compilers/d: Support linker selection with gdc

This should have worked before, but the inheritance order was backwards,
so we got the DCompiler before the GnuCompiler, and the base Compiler
methods overrode the Gnu methods.

mesonbuild/compilers/d.py

@@ -627,7 +627,7 @@ class DCompiler(Compiler):
         return ' '.join(self.exelist)
 
 
-class GnuDCompiler(DCompiler, GnuCompiler):
+class GnuDCompiler(GnuCompiler, DCompiler):
 
     # we mostly want DCompiler, but that gives us the Compiler.LINKER_PREFIX instead
     LINKER_PREFIX = GnuCompiler.LINKER_PREFIX