This patch reverts upstream commit b4d3485e4fc1d:

  https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=b4d3485e4fc1d029e620a59deb54b3f4f3f6b209

Otherwise, GCC ends up searching the wrong target directory for cross-headers.

diff --git a/gcc/configure b/gcc/configure
--- a/gcc/configure
+++ b/gcc/configure
@@ -3689,11 +3689,6 @@ esac
 fi
 
 
-# If both --with-sysroot and --with-gxx-include-dir are passed, we interpolate
-# the former in the latter and, upon success, compute gcc_gxx_include_dir as
-# relative to the sysroot.
-gcc_gxx_include_dir_add_sysroot=0
-
 # This logic must match libstdc++-v3/acinclude.m4:GLIBCXX_EXPORT_INSTALL_INFO.
 if test x${gcc_gxx_include_dir} = x; then
   if test x${enable_version_specific_runtime_libs} = xyes; then
@@ -3705,10 +3700,15 @@ if test x${gcc_gxx_include_dir} = x; then
     fi
     gcc_gxx_include_dir="\$(libsubdir)/\$(libsubdir_to_prefix)$libstdcxx_incdir"
   fi
-elif test "${with_sysroot+set}" = set; then
+fi
+
+gcc_gxx_include_dir_add_sysroot=0
+if test "${with_sysroot+set}" = set; then
   gcc_gxx_without_sysroot=`expr "${gcc_gxx_include_dir}" : "${with_sysroot}"'\(.*\)'`
   if test "${gcc_gxx_without_sysroot}"; then
-    gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    if test x${with_sysroot} != x/; then
+      gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    fi
     gcc_gxx_include_dir_add_sysroot=1
   fi
 fi
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 715fcba0482..88136e232df 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -204,11 +204,6 @@ no)	;;
 *)	gcc_gxx_include_dir=$with_gxx_include_dir ;;
 esac])
 
-# If both --with-sysroot and --with-gxx-include-dir are passed, we interpolate
-# the former in the latter and, upon success, compute gcc_gxx_include_dir as
-# relative to the sysroot.
-gcc_gxx_include_dir_add_sysroot=0
-
 # This logic must match libstdc++-v3/acinclude.m4:GLIBCXX_EXPORT_INSTALL_INFO.
 if test x${gcc_gxx_include_dir} = x; then
   if test x${enable_version_specific_runtime_libs} = xyes; then
@@ -220,10 +215,15 @@ if test x${gcc_gxx_include_dir} = x; then
     fi
     gcc_gxx_include_dir="\$(libsubdir)/\$(libsubdir_to_prefix)$libstdcxx_incdir"
   fi
-elif test "${with_sysroot+set}" = set; then
+fi
+
+gcc_gxx_include_dir_add_sysroot=0
+if test "${with_sysroot+set}" = set; then
   gcc_gxx_without_sysroot=`expr "${gcc_gxx_include_dir}" : "${with_sysroot}"'\(.*\)'`
   if test "${gcc_gxx_without_sysroot}"; then
-    gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    if test x${with_sysroot} != x/; then
+      gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    fi
     gcc_gxx_include_dir_add_sysroot=1
   fi
 fi