By default commands such as "ctest" would not look for certificates
at all: <https://issues.guix.gnu.org/issue/37371>.

This changes CMake such that commands honor SSL_CERT_FILE and SSL_CERT_DIR
as well as /etc/ssl/certs.

--- cmake-3.13.1/Source/cmCurl.cxx	2019-09-10 17:27:36.926907260 +0200
+++ cmake-3.13.1/Source/cmCurl.cxx	2019-09-10 17:52:35.475903919 +0200
@@ -2,11 +2,8 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #include "cmCurl.h"
 
-#if !defined(CMAKE_USE_SYSTEM_CURL) && !defined(_WIN32) &&                    \
-  !defined(__APPLE__) && !defined(CURL_CA_BUNDLE) && !defined(CURL_CA_PATH)
 #  define CMAKE_FIND_CAFILE
 #  include "cmSystemTools.h"
-#endif
 #include "cmStringAlgorithms.h"
 
 // curl versions before 7.21.5 did not provide this error code
@@ -30,6 +27,19 @@
     ::CURLcode res = ::curl_easy_setopt(curl, CURLOPT_CAINFO, cafile);
     check_curl_result(res, "Unable to set TLS/SSL Verify CAINFO: ");
   }
+
+  /* Honor the usual environment variables.  */
+  else if (cmSystemTools::GetEnv("SSL_CERT_FILE", e)) {
+    ::CURLcode res =
+      ::curl_easy_setopt(curl, CURLOPT_CAINFO, e.c_str());
+    check_curl_result(res, "Unable to set TLS/SSL Verify CAINFO: ");
+  }
+  else if (cmSystemTools::GetEnv("SSL_CERT_DIR", e)) {
+    ::CURLcode res =
+      ::curl_easy_setopt(curl, CURLOPT_CAPATH, e.c_str());
+    check_curl_result(res, "Unable to set TLS/SSL Verify CAINFO: ");
+  }
+
 #ifdef CMAKE_FIND_CAFILE
 #  define CMAKE_CAFILE_FEDORA "/etc/pki/tls/certs/ca-bundle.crt"
   else if (cmSystemTools::FileExists(CMAKE_CAFILE_FEDORA, true)) {