[CMake] detecting __attribute__((weak)) support

[Laurent Demailly]

I searched and didn't find yet ­ would it be reasonable to have built in
support in cmake for knowing if the platform/compiler supports weak
symbols ?

I'm porting the following autconf fragment:

AC_CACHE_CHECK(
  [for weak symbol support],
  [folly_cv_prog_cc_weak_symbols],
  [AC_LINK_IFELSE(
    [AC_LANG_SOURCE[
      extern "C" void configure_link_extern_weak_test()
__attribute__((weak));
      int main(int argc, char** argv) {
          return configure_link_extern_weak_test == nullptr;
      }]],
    [folly_cv_prog_cc_weak_symbols="yes"],
    [folly_cv_prog_cc_weak_symbols="no"])])

if test "$folly_cv_prog_cc_weak_symbols" = yes; then
  AC_DEFINE([HAVE_WEAK_SYMBOLS], [1],
            [Define to 1 if the linker supports weak symbols.])
fi

And I'm not sure what's the best way to do such recipe in Cmake ?
(by hand for now pending possible support in 3.2+ maybe)


Laurent