build/meson: only add WINPTHREAD_COND_DECL workaround if winpthreads is found

Author: mochaaP

meson.build

@@ -61,11 +61,12 @@ msft_proxy4_dep = declare_dependency(
 modules = get_option('modules')
 modules_interface_spec = {
   'gcc': {
-    'flags': [
-      '-fmodules',
-      '-fdeps-format=p1689r5',
+    'flags': ['-fmodules', '-fdeps-format=p1689r5'] + (cxx.has_header_symbol(
+      'pthread.h',
+      '__WINPTHREADS_VERSION',
+    ) ? [
       '-DWINPTHREAD_COND_DECL=WINPTHREADS_ALWAYS_INLINE',
-    ],
+    ] : []),
     'check_only': ['-M'],
   },
   'msvc': {