assert: choose function-var name on non-gnu

Selects __PRETTY_FUNCTION__ or __func__. Linux assumes GNU, and chooses
__PRETTY_FUNCTION__ if gcc/g++ versions are favorable.

This also includes a fix in ax_c_var_func.m4:

AC_TRY_COMPILE will wrap the test in main{}, and then GCC will complain
about nested functions. Just use the original main{} body.

diff --git a/m4/ax_c_var_func.m4 b/m4/ax_c_var_func.m4
index 0ad7d2b..8b57563 100644
--- a/m4/ax_c_var_func.m4
+++ b/m4/ax_c_var_func.m4
@@ -57,9 +57,9 @@ AC_DEFUN([AX_C_VAR_FUNC],
 [AC_REQUIRE([AC_PROG_CC])
 AC_CACHE_CHECK(whether $CC recognizes __func__, ac_cv_c_var_func,
 AC_TRY_COMPILE(,
-[int main() {
+[
 char *s = __func__;
-}],
+],
 AC_DEFINE(HAVE_FUNC,,
 [Define if the C complier supports __func__]) ac_cv_c_var_func=yes,
 ac_cv_c_var_func=no) )

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>