meson.build: Fix detection of symbols

This patch changes the behavior of meson to define configuration options
*only* when the symbol checked is present. Currently, it defines all of
them in config.h whether the symbol exists or not and the code that
looks for it doesn't check the macro's value, but whether it's defined.

Signed-off-by: Mark Weiman <mark.weiman@markzz.com>
Signed-off-by: Allan McRae <allan@archlinux.org>

meson.build

@@ -146,7 +146,9 @@ foreach sym : [
     'tcflush',
   ]
   have = cc.has_function(sym, args : '-D_GNU_SOURCE')
+  if have
     conf.set10('HAVE_' + sym.to_upper(), have)
+  endif
 endforeach
 
 foreach member : [