Allow spirv-tools and glslang to live elsewhere.

This allows both spirv-tools and glslang to live in separate source
directories when being invoked with ndk-build.

ndk-build THIRD_PARTY_PATH=foo  specifies that foo/glslang and
foo/spirv-tools exist.

ndk-build SPVTOOLS_LOCAL_PATH=/some/absolute/path specifies that glslang
exists in third_party/glslang but spirv-tools exists at
/some/absolute/path.

ndk-build GLSLANG_LOCAL_PATH=/some/absolute/path is the same but for
glslang.