This patch replaces the OS-specific detection mechanism by one that solely
relies on environment variables.  This has the benefit that said environment
variables can be used as search paths in Guix.

diff --git a/lib/std/zig/system/NativePaths.zig b/lib/std/zig/system/NativePaths.zig
index 8e3e46e48..1ed9d3206 100644
--- a/lib/std/zig/system/NativePaths.zig
+++ b/lib/std/zig/system/NativePaths.zig
@@ -26,73 +26,42 @@ pub fn detect(allocator: Allocator, native_info: NativeTargetInfo) !NativePaths
     };
     errdefer self.deinit();
 
-    var is_nix = false;
-    if (process.getEnvVarOwned(allocator, "NIX_CFLAGS_COMPILE")) |nix_cflags_compile| {
-        defer allocator.free(nix_cflags_compile);
-
-        is_nix = true;
-        var it = mem.tokenize(u8, nix_cflags_compile, " ");
+    // TODO: Support cross-compile paths?
+    if (process.getEnvVarOwned(allocator, "C_INCLUDE_PATH")) |c_include_path| {
+        defer allocator.free(c_include_path);
+        var it = mem.tokenize(u8, c_include_path, ":");
         while (true) {
-            const word = it.next() orelse break;
-            if (mem.eql(u8, word, "-isystem")) {
-                const include_path = it.next() orelse {
-                    try self.addWarning("Expected argument after -isystem in NIX_CFLAGS_COMPILE");
-                    break;
-                };
-                try self.addIncludeDir(include_path);
-            } else {
-                if (mem.startsWith(u8, word, "-frandom-seed=")) {
-                    continue;
-                }
-                try self.addWarningFmt("Unrecognized C flag from NIX_CFLAGS_COMPILE: {s}", .{word});
-            }
+            const dir = it.next() orelse break;
+            try self.addIncludeDir(dir);
         }
     } else |err| switch (err) {
         error.InvalidUtf8 => {},
         error.EnvironmentVariableNotFound => {},
         error.OutOfMemory => |e| return e,
     }
-    if (process.getEnvVarOwned(allocator, "NIX_LDFLAGS")) |nix_ldflags| {
-        defer allocator.free(nix_ldflags);
-
-        is_nix = true;
-        var it = mem.tokenize(u8, nix_ldflags, " ");
+    if (process.getEnvVarOwned(allocator, "CPLUS_INCLUDE_PATH")) |cplus_include_path| {
+        defer allocator.free(cplus_include_path);
+        var it = mem.tokenize(u8, cplus_include_path, ":");
         while (true) {
-            const word = it.next() orelse break;
-            if (mem.eql(u8, word, "-rpath")) {
-                const rpath = it.next() orelse {
-                    try self.addWarning("Expected argument after -rpath in NIX_LDFLAGS");
-                    break;
-                };
-                try self.addRPath(rpath);
-            } else if (word.len > 2 and word[0] == '-' and word[1] == 'L') {
-                const lib_path = word[2..];
-                try self.addLibDir(lib_path);
-            } else {
-                try self.addWarningFmt("Unrecognized C flag from NIX_LDFLAGS: {s}", .{word});
-                break;
-            }
+            const dir = it.next() orelse break;
+            try self.addIncludeDir(dir);
         }
     } else |err| switch (err) {
         error.InvalidUtf8 => {},
         error.EnvironmentVariableNotFound => {},
         error.OutOfMemory => |e| return e,
     }
-    if (is_nix) {
-        return self;
-    }
-
-    if (comptime builtin.target.isDarwin()) {
-        try self.addIncludeDir("/usr/include");
-        try self.addIncludeDir("/usr/local/include");
-
-        try self.addLibDir("/usr/lib");
-        try self.addLibDir("/usr/local/lib");
-
-        try self.addFrameworkDir("/Library/Frameworks");
-        try self.addFrameworkDir("/System/Library/Frameworks");
-
-        return self;
+    if (process.getEnvVarOwned(allocator, "LIBRARY_PATH")) |library_path| {
+        defer allocator.free(library_path);
+        var it = mem.tokenize(u8, library_path, ":");
+        while (true) {
+            const dir = it.next() orelse break;
+            try self.addLibDir(dir);
+        }
+    } else |err| switch (err) {
+        error.InvalidUtf8 => {},
+        error.EnvironmentVariableNotFound => {},
+        error.OutOfMemory => |e| return e,
     }
 
     if (comptime native_target.os.tag == .solaris) {
@@ -106,32 +75,17 @@ pub fn detect(allocator: Allocator, native_info: NativeTargetInfo) !NativePaths
         return self;
     }
 
-    if (native_target.os.tag != .windows) {
-        const triple = try native_target.linuxTriple(allocator);
-        const qual = native_target.cpu.arch.ptrBitWidth();
-
-        // TODO: $ ld --verbose | grep SEARCH_DIR
-        // the output contains some paths that end with lib64, maybe include them too?
-        // TODO: what is the best possible order of things?
-        // TODO: some of these are suspect and should only be added on some systems. audit needed.
-
-        try self.addIncludeDir("/usr/local/include");
-        try self.addLibDirFmt("/usr/local/lib{d}", .{qual});
-        try self.addLibDir("/usr/local/lib");
-
-        try self.addIncludeDirFmt("/usr/include/{s}", .{triple});
-        try self.addLibDirFmt("/usr/lib/{s}", .{triple});
-
-        try self.addIncludeDir("/usr/include");
-        try self.addLibDirFmt("/lib{d}", .{qual});
-        try self.addLibDir("/lib");
-        try self.addLibDirFmt("/usr/lib{d}", .{qual});
-        try self.addLibDir("/usr/lib");
-
-        // example: on a 64-bit debian-based linux distro, with zlib installed from apt:
-        // zlib.h is in /usr/include (added above)
-        // libz.so.1 is in /lib/x86_64-linux-gnu (added here)
-        try self.addLibDirFmt("/lib/{s}", .{triple});
+    if (process.getEnvVarOwned(allocator, "DYLD_FRAMEWORK_PATH")) |dyld_framework_path| {
+        defer allocator.free(dyld_framework_path);
+        var it = mem.tokenize(u8, dyld_framework_path, ":");
+        while (true) {
+            const dir = it.next() orelse break;
+            try self.addFrameworkDir(dir);
+        }
+    } else |err| switch (err) {
+        error.InvalidUtf8 => {},
+        error.EnvironmentVariableNotFound => {},
+        error.OutOfMemory => |e| return e,
     }
 
     return self;