Fix CXX/Fortran MODULE flags when enabled before C (#12929)

If CXX or Fortran is enabled before C then the values of

  CMAKE_SHARED_MODULE_C_FLAGS
  CMAKE_SHARED_MODULE_CREATE_C_FLAGS

may not be available.  On platforms where MODULE library (plugin) creation
is the same as SHARED library creation initialize the MODULE creation
flags from the SHARED creation flags of the matching language instead of
assuming that C has been enabled first.

Teach the COnly and CxxOnly tests to build MODULE libraries.  The latter
covers this specific case.

1 files changed, 6 insertions, 0 deletions

diff --git a/Tests/COnly/testCModule.c b/Tests/COnly/testCModule.c
new file mode 100644
index 0000000..1a89292
--- /dev/null
+++ b/Tests/COnly/testCModule.c
@@ -0,0 +1,6 @@
+#ifdef _WIN32
+# define TEST_EXPORT __declspec(dllexport)
+#else
+# define TEST_EXPORT
+#endif
+TEST_EXPORT int testCModule(void) { return 0; }