iconv dependency: include header when checking for libc builtin

This header is required anyway. And the compile test for linking to libc
with the iconv_open symbol, can succeed when we try to use the literal
symbol name without includes, but fail later during project build,
because actually including iconv.h might redefine the function to match
a forked symbol. This happens when GNU iconv is installed as a
standalone library on systems that have a less fully-featured iconv
implementation.

So, by including the header in has_function, we ensure that we test
against the default resolved header. In the event that the symbol which
is #define'd by the header is 'libiconv_open', linking will fail against
libc even when a builtin iconv does exist, and we will fall back to the
iconv dependency that provides -liconv (and which is needed to properly
use the default header).

Fixes compiling against the iconv dependency on e.g. FreeBSD when the
libiconv port is installed.

mesonbuild/dependencies/misc.py

@@ -452,7 +452,7 @@ class IconvBuiltinDependency(BuiltinDependency):
     def __init__(self, name: str, env: 'Environment', kwargs: T.Dict[str, T.Any]):
         super().__init__(name, env, kwargs)
 
-        if self.clib_compiler.has_function('iconv_open', '', env)[0]:
+        if self.clib_compiler.has_function('iconv_open', '#include <iconv.h>', env)[0]:
             self.is_found = True