Works when mixing Java and C.

mesonbuild/backend/backends.py

@@ -203,7 +203,10 @@ class Backend():
                 if c.can_compile(s):
                     return cpp
         for c in self.build.compilers:
-            if c.get_language() != 'vala':
+            if c.get_language() == 'vala':
+                continue
+            for s in src:
+                if c.can_compile(s):
                     return c
         raise RuntimeError('Unreachable code')