Make G_STRFUNC consistent across platforms

Make G_STRFUNC consistent across platforms

Recent versions of clang have changed __PRETTY_FUNCTION__ to always
include the function signature (rather than including the function
signature in C++ but not in C like gcc does). This causes G_STRFUNC to
give different results under clang and gcc, causing some tests with
g_test_expect_messages() to fail.

Fix this by only using __PRETTY_FUNCTION__ in C++, and using
__FUNCTION__ in C. (Under gcc this change has no effect.)

https://bugzilla.gnome.org/show_bug.cgi?id=702147