Fix windows tests when using newer MinGW.

environment.py

@@ -77,7 +77,7 @@ class Environment():
 
         # List of potential compilers.
         if mesonlib.is_windows():
-            self.default_c = ['cl', 'cc']
+            self.default_c = ['cl', 'cc', 'gcc']
             self.default_cpp = ['cl', 'c++']
         else:
             self.default_c = ['cc']

run_tests.py

@@ -288,7 +288,13 @@ def generate_prebuilt_object():
             objectfile = objectbase + 'obj'
         else:
             objectfile = objectbase + 'o'
-        cmd = ['cc', '-c', source, '-o', objectfile]
+        if shutil.which('cc'):
+            cmd = 'cc'
+        elif shutil.which('gcc'):
+            cmd = 'gcc'
+        else:
+            raise RuntimeError("Could not find C compiler.")
+        cmd = [cmd, '-c', source, '-o', objectfile]
     subprocess.check_call(cmd)
     return objectfile