backend/vs: Fix OpenMP support

Use the IDE's OpenMP flag instead of adding /openmp to additional
arguments. The IDE appears to override /openmp in additional arguments
with the IDE setting, which defaults to false, leading to binaries built
without OpenMP.

mesonbuild/backend/vs2010backend.py

@@ -992,7 +992,7 @@ class Vs2010Backend(backends.Backend):
             # Cflags required by external deps might have UNIX-specific flags,
             # so filter them out if needed
             if isinstance(d, dependencies.OpenMPDependency):
-                d_compile_args = compiler.openmp_flags()
+                ET.SubElement(clconf, 'OpenMPSupport').text = 'true'
             else:
                 d_compile_args = compiler.unix_args_to_native(d.get_compile_args())
                 for arg in d_compile_args:
@@ -1100,14 +1100,14 @@ class Vs2010Backend(backends.Backend):
                 # Extend without reordering or de-dup to preserve `-L -l` sets
                 # https://github.com/mesonbuild/meson/issues/1718
                 if isinstance(dep, dependencies.OpenMPDependency):
-                    extra_link_args.extend_direct(compiler.openmp_flags())
+                    ET.SubElement(clconf, 'OpenMPSuppport').text = 'true'
                 else:
                     extra_link_args.extend_direct(dep.get_link_args())
             for d in target.get_dependencies():
                 if isinstance(d, build.StaticLibrary):
                     for dep in d.get_external_deps():
                         if isinstance(dep, dependencies.OpenMPDependency):
-                            extra_link_args.extend_direct(compiler.openmp_flags())
+                            ET.SubElement(clconf, 'OpenMPSuppport').text = 'true'
                         else:
                             extra_link_args.extend_direct(dep.get_link_args())
         # Add link args for c_* or cpp_* build options. Currently this only