build: Disable gcc warnings as warnings, too

We use -Werror in the build, so even if some warnings are just warnings,
they'd be errors.

demos/gtk-demo/meson.build

@@ -228,7 +228,9 @@ endif
 # Use a subset of compiler flags
 demo_cflags = []
 foreach flag: common_cflags
-  if flag not in ['-Werror=missing-prototypes', '-Werror=missing-declarations', '-fvisibility=hidden']
+  if flag not in ['-Werror=missing-prototypes', '-Wmissing-prototypes',
+                  '-Werror=missing-declarations', '-Wmissing-declarations',
+                  '-fvisibility=hidden']
     demo_cflags += flag
   endif
 endforeach

testsuite/a11y/meson.build

@@ -44,7 +44,9 @@ is_debug = get_option('buildtype').startswith('debug')
 test_cargs = []
 
 foreach flag: common_cflags
-  if flag not in ['-Werror=missing-prototypes', '-Werror=missing-declarations', '-fvisibility=hidden']
+  if flag not in ['-Werror=missing-prototypes', '-Wmissing-prototypes',
+                  '-Werror=missing-declarations', '-Wmissing-declarations',
+                  '-fvisibility=hidden']
     test_cargs += flag
   endif
 endforeach

testsuite/gtk/meson.build

@@ -152,7 +152,9 @@ if os_unix
 endif
 
 foreach flag: common_cflags
-  if flag not in ['-Werror=missing-prototypes', '-Werror=missing-declarations', '-fvisibility=hidden']
+  if flag not in ['-Werror=missing-prototypes', '-Wmissing-prototypes',
+                  '-Werror=missing-declarations', '-Wmissing-declarations',
+                  '-fvisibility=hidden']
     test_cargs += flag
   endif
 endforeach

testsuite/reftests/meson.build

@@ -2,7 +2,9 @@
 
 reftest_cflags = []
 foreach flag: common_cflags
-  if flag not in ['-Werror=missing-prototypes', '-Werror=missing-declarations', '-fvisibility=hidden']
+  if flag not in ['-Werror=missing-prototypes', '-Wmissing-prototypes',
+                  '-Werror=missing-declarations', '-Wmissing-declarations',
+                  '-fvisibility=hidden']
     reftest_cflags += flag
   endif
 endforeach