travis-ci: update build matrix

travis-ci: update build matrix

Add clang-3.6/x86_64, clang-3.6/x86, and musl-gcc/x86_64 to the matrix.

* .travis.yml (compiler): Add gcc, clang-3.6, and musl-gcc.
(env): Add matrix, remove global ENABLE_GCC_WERROR.
(matrix): Add exclude, remove include.
* travis-ci.sh: Add support for clang and musl-gcc.
Use --enable-gcc-Werror for gcc build unconditionally.