remove exec flag on source files.
Add `scanner_module` as compiler option.
Check OS environment for additional compiler options.

Any options found in the "ERLYDTL_COMPILER_OPTIONS" env variable will
be appended to the options passed to erlydtl.

For instance, to use this to run the test suite with a non-default scanner::

  $ ERLYDTL_COMPILER_OPTIONS={scanner_module,erlydtl_new_scanner} make test