Implement thread linker args for D compilers

D compilers are configured to have highest priority when chosing linker
for targets mixing C/C++/D code and before this change meson would fail
to configure gtest target that uses D library as a dependency.

mesonbuild/compilers/d.py

@@ -409,6 +409,9 @@ class DCompiler(Compiler):
     def get_crt_link_args(self, crt_val, buildtype):
         return []
 
+    def thread_link_flags(self, env):
+        return ['-pthread']
+
 class GnuDCompiler(DCompiler):
     def __init__(self, exelist, version, is_cross, arch, **kwargs):
         DCompiler.__init__(self, exelist, version, is_cross, arch, **kwargs)