Please see the patch. Declaring functions in AC_TRY_LINK and AC_TRY_COMPILE causes the tests to fail on Clang.

Both AC_TRY_LINK and AC_TRY_COMPILE put code to test in the main function, so instead of using 'int main (void) { return 0; }' as test program, we must use 'return 0' without declaring any function. If we declare main function there, it becomes a nested main function inside the global main function. It is supported by GCC, but not by Clang.

This causes problems when checking compiler flags:

checking for -Bsymbolic-functions linker flag... no
checking for -fvisibility=hidden compiler flag... no