build: fix compilation with mingw-w64/Clang

This fixes the build in mingw-w64/Clang on MSYS2. It also disables the
use of gnu_printf in Clang, which was what was causing most of the
warnings. The Clang-compiled mpv binary appears to work, but there are
no guarantees yet, since until now mpv has only been tested with
mingw-w64/GCC on Windows.

Fixes #3800

osdep/compiler.h

@@ -10,7 +10,7 @@
 /** Use gcc attribute to check printf fns.  a1 is the 1-based index of
  * the parameter containing the format, and a2 the index of the first
  * argument. **/
-#ifdef __MINGW32__
+#if defined(__MINGW32__) && !defined(__clang__)
 // MinGW maps "printf" to the non-standard MSVCRT functions, even if
 // __USE_MINGW_ANSI_STDIO is defined and set to 1. We need to use "gnu_printf",
 // which isn't necessarily available on other GCC compatible compilers.

ta/ta.h

@@ -27,7 +27,7 @@
 #endif
 
 // Broken crap with __USE_MINGW_ANSI_STDIO
-#ifdef __MINGW32__
+#if defined(__MINGW32__) && defined(__GNUC__) && !defined(__clang__)
 #undef TA_PRF
 #define TA_PRF(a1, a2) __attribute__ ((format (gnu_printf, a1, a2)))
 #endif

wscript

@@ -119,7 +119,7 @@ main_dependencies = [
         'name': 'mingw',
         'desc': 'MinGW',
         'deps': [ 'os-win32' ],
-        'func': check_statement('stddef.h', 'int x = __MINGW32__;'
+        'func': check_statement('stdlib.h', 'int x = __MINGW32__;'
                                             'int y = __MINGW64_VERSION_MAJOR'),
     }, {
         'name': 'posix',