Refactor and improve test Makefile

Removes duplication and cleanup output.
This greatly simplifies the reusing of this makefile from
other Cesanta C projects.

Added `compile` target which performs a blazingly fast parallel syntax only
check for multiple dialects. As the tests take longer and longer, it's becoming
frustrating to figure out that there were a syntax violation in one dialect
after 30 seconds have passed.
Merge pull request #170 from cesanta/cleanup_test_mk

Refactor and improve test Makefile