Add support for extra modules and custom compilers to test-build.

With this change we can now use test-build for Jenkins.

## Extra Modules

Set the `TEST_BUILD_MODULES` environment variable to a list of modules to test. For example:

    export TEST_BUILD_MODULES="m_ssl_gnutls.cpp,m_ssl_openssl.cpp"
    ./tools/test-build

## Custom Compilers

Pass extra compilers to test-build as command line arguments. For example:

    ./tools/test-build clang++-3.5 g++-4.9