GNUC: also check __GNUG__ define when checking for g++

At least the scratchbox compiler for the N900, which basically is a gcc 4.2,
sets only __GNUG__ in C++ mode, but not __GNUC__. It does indeed set
__GNUC_MINOR__ and __GNUC_PATCHLEVEL__. Extend the compiler detection code for
GNU compilers in C++ mode to look at __GNUG__ in case __GNUC__ is absent.

Modules/Compiler/GNU-CXX-DetermineCompiler.cmake

+
+set(_compiler_id_pp_test "defined(__GNUC__) || defined(__GNUG__)")
+
+set(_compiler_id_version_compute "
+# if defined(__GNUC__)
+#  define @PREFIX@COMPILER_VERSION_MAJOR @MACRO_DEC@(__GNUC__)
+# else
+#  define @PREFIX@COMPILER_VERSION_MAJOR @MACRO_DEC@(__GNUG__)
+# endif
+# if defined(__GNUC_MINOR__)
+#  define @PREFIX@COMPILER_VERSION_MINOR @MACRO_DEC@(__GNUC_MINOR__)
+# endif
+# if defined(__GNUC_PATCHLEVEL__)
+#  define @PREFIX@COMPILER_VERSION_PATCH @MACRO_DEC@(__GNUC_PATCHLEVEL__)
+# endif")