meson: use project instead of global arguments

It makes sense for libplacebo to be a subproject of some other project,
but for that to be possible you're not allowed to use the global
arguments here (meson will fail and complain). Just use the project
variant instead which plays nice.

meson.build

@@ -26,7 +26,7 @@ if git.found()
 endif
 
 
-# Global build options
+# Project build options
 build_opts = [
   # Warnings
   '-Wundef', '-Wshadow', '-Wparentheses', '-Wpointer-arith',
@@ -69,9 +69,9 @@ if cc.has_link_argument(vflag)
   link_args += [vflag]
 endif
 
-add_global_arguments(build_opts + c_opts, language: 'c')
-add_global_arguments(build_opts + cpp_opts, language: 'cpp')
-add_global_link_arguments(link_args, language: 'c')
+add_project_arguments(build_opts + c_opts, language: 'c')
+add_project_arguments(build_opts + cpp_opts, language: 'cpp')
+add_project_link_arguments(link_args, language: 'c')
 
 subdir('src')