msvc: Avoid spurious openmp link warnings

The linker that comes with MSVC does not understand the /openmp flag.
This results in a string of

    LINK : warning LNK4044: unrecognized option '/openmp'; ignored

warnings, one for each static_library linked with an executable.

Avoid this by only setting the linker openmp flag when the compiler is
not MSVC.

mesonbuild/compilers/compilers.py

@@ -1087,6 +1087,9 @@ class Compiler:
     def openmp_flags(self):
         raise EnvironmentException('Language %s does not support OpenMP flags.' % self.get_display_language())
 
+    def openmp_link_flags(self):
+        return self.openmp_flags()
+
     def language_stdlib_only_link_flags(self):
         return []
 

mesonbuild/compilers/mixins/visualstudio.py

@@ -208,6 +208,9 @@ class VisualStudioLikeCompiler(metaclass=abc.ABCMeta):
     def openmp_flags(self) -> T.List[str]:
         return ['/openmp']
 
+    def openmp_link_flags(self) -> T.List[str]:
+        return []
+
     # FIXME, no idea what these should be.
     def thread_flags(self, env: 'Environment') -> T.List[str]:
         return []

mesonbuild/dependencies/misc.py

@@ -97,7 +97,8 @@ class OpenMPDependency(ExternalDependency):
             for name in header_names:
                 if self.clib_compiler.has_header(name, '', self.env, dependencies=[self], disable_cache=True)[0]:
                     self.is_found = True
-                    self.compile_args = self.link_args = self.clib_compiler.openmp_flags()
+                    self.compile_args = self.clib_compiler.openmp_flags()
+                    self.link_args = self.clib_compiler.openmp_link_flags()
                     break
             if not self.is_found:
                 mlog.log(mlog.yellow('WARNING:'), 'OpenMP found but omp.h missing.')