Explicitly check for OpenMP headers.

mesonbuild/compilers/compilers.py

@@ -1282,7 +1282,8 @@ class ClangCompiler:
         elif version_compare(self.version, '>=3.7.0'):
             return ['-fopenmp=libomp']
         else:
-            return None
+            # Shouldn't work, but it'll be checked explicitly in the OpenMP dependency.
+            return []
 
 
 # Tested on linux for ICC 14.0.3, 15.0.6, 16.0.4, 17.0.1

mesonbuild/dependencies/misc.py

@@ -252,10 +252,18 @@ class OpenMPDependency(ExternalDependency):
     def __init__(self, environment, kwargs):
         language = kwargs.get('language')
         super().__init__('openmp', environment, language, kwargs)
-        self.is_found = True
+        self.is_found = False
         openmp_date = self.compiler.get_define('_OPENMP', '', self.env, [], [self])
-        self.version = self.VERSIONS[openmp_date]
-        mlog.log('Dependency', mlog.bold(self.name), 'found:', mlog.green('YES'), self.version)
+        if openmp_date:
+            self.version = self.VERSIONS[openmp_date]
+            if self.compiler.has_header('omp.h', '', self.env, dependencies=[self]):
+                self.is_found = True
+            else:
+                mlog.log(mlog.yellow('WARNING:'), 'OpenMP found but omp.h missing.')
+        if self.is_found:
+            mlog.log('Dependency', mlog.bold(self.name), 'found:', mlog.green('YES'), self.version)
+        else:
+            mlog.log('Dependency', mlog.bold(self.name), 'found:', mlog.red('NO'))
 
     def need_openmp(self):
         return True