GenerateExportHeader: use double quotes around _gcc_version

The GenerateExportHeaders test was failing on one machine, the version
could not be determined there, so the _gcc_version was empty,
so the first argument to if() was empty, so it complained:
http://open.cdash.org/testDetails.php?test=135623436&build=2016288
Use double quotes to turn the non-existant first argument into an empty
string.

Alex

Modules/GenerateExportHeader.cmake

@@ -173,7 +173,7 @@ macro(_test_compiler_hidden_visibility)
         _gcc_version "${_gcc_version_info}")
     endif()
 
-    if(${_gcc_version} VERSION_LESS "4.2")
+    if("${_gcc_version}" VERSION_LESS "4.2")
       set(GCC_TOO_OLD TRUE)
       message(WARNING "GCC version older than 4.2")
     endif()