bugfix: don't fail if checking for "real" compiler version

doesn't understand a custom, user-defined compiler version.  However, if
the compiler's version check fails, you can't build anything with the
custom compiler.

- [x] Be more lenient: fall back to the custom compiler version and use
  it verbatim if the version check fails.

lib/spack/spack/architecture.py

@@ -69,6 +69,7 @@
 import spack.compiler
 import spack.paths
 import spack.error as serr
+import spack.util.executable
 import spack.version
 from spack.util.naming import mod_to_class
 from spack.util.spack_yaml import syaml_dict
@@ -214,7 +215,11 @@ def optimization_flags(self, compiler):
             import spack.spec
             if isinstance(compiler, spack.spec.CompilerSpec):
                 compiler = spack.compilers.compilers_for_spec(compiler).pop()
-            compiler_version = compiler.cc_version(compiler.cc)
+            try:
+                compiler_version = compiler.cc_version(compiler.cc)
+            except spack.util.executable.ProcessError as e:
+                # log this and just return compiler.version instead
+                tty.debug(str(e))
 
         return self.microarchitecture.optimization_flags(
             compiler.name, str(compiler_version)