GenerateExportHeader: improve compiler identification

Use CMAKE_<LANG>_COMPILER_VERSION instead of calling the compiler. This macro
predates those useful variables. This also fixes the issue that g++ version
detection was not working if C language was not enabled.

Modules/GenerateExportHeader.cmake

@@ -154,34 +154,15 @@ endmacro()
 
 macro(_test_compiler_hidden_visibility)
 
-  if(CMAKE_COMPILER_IS_GNUCXX)
-    exec_program(${CMAKE_C_COMPILER} ARGS --version
-      OUTPUT_VARIABLE _gcc_version_info)
-    string(REGEX MATCH "[345]\\.[0-9]\\.[0-9]"
-      _gcc_version "${_gcc_version_info}")
-    # gcc on mac just reports: "gcc (GCC) 3.3 20030304 ..." without the
-    # patch level, handle this here:
-    if(NOT _gcc_version)
-      string(REGEX REPLACE ".*\\(GCC\\).* ([34]\\.[0-9]) .*" "\\1.0"
-        _gcc_version "${_gcc_version_info}")
-    endif()
-
-    if("${_gcc_version}" VERSION_LESS "4.2")
-      set(GCC_TOO_OLD TRUE)
-      message(WARNING "GCC version older than 4.2")
-    endif()
-  endif()
-
-  if(CMAKE_CXX_COMPILER_ID MATCHES Intel)
-    exec_program(${CMAKE_CXX_COMPILER} ARGS -V
-      OUTPUT_VARIABLE _intel_version_info)
-    string(REGEX REPLACE ".*Version ([0-9]+(\\.[0-9]+)+).*" "\\1"
-      _intel_version "${_intel_version_info}")
-
-    if(${_intel_version} VERSION_LESS "12.0")
-      set(_INTEL_TOO_OLD TRUE)
-      message(WARNING "Intel compiler older than 12.0")
-    endif()
+  if(CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.2")
+    set(GCC_TOO_OLD TRUE)
+    message(WARNING "GCC version older than 4.2")
+  elseif(CMAKE_COMPILER_IS_GNUC AND CMAKE_C_COMPILER_VERSION VERSION_LESS "4.2")
+    set(GCC_TOO_OLD TRUE)
+    message(WARNING "GCC version older than 4.2")
+  elseif(CMAKE_CXX_COMPILER_ID MATCHES Intel AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "12.0")
+    set(_INTEL_TOO_OLD TRUE)
+    message(WARNING "Intel compiler older than 12.0")
   endif()