Fix building with GCC

GCC does not know the clang diagnostic pragma.

meson.build

@@ -2,7 +2,7 @@ project('pi2-view', ['cpp', 'c'],
     license: 'GPL3',
     meson_version: '>=0.45.1',
     version: '0.1',
-    default_options : ['warning_level=3', 'werror=true', 'cpp_std=c++17', 'c_std=c11']
+    default_options : ['warning_level=3', 'cpp_args=-Wno-unknown-pragmas', 'werror=true', 'cpp_std=c++17', 'c_std=c11']
 )
 
 microsoft_gsl_proj = subproject('microsoft-gsl')