Prior to this commit, externally defined RC target files, like
`sac2crc.backend.cuda', where not being handled correctly by autotools,
resulting in `sac2crc' containing targets with missing arguments. In essence,
the problem is due to autotools *not* substituting variables with their values
into the RC files before being merged together to generate `sac2crc.pre'. This
commit fixes this by adding the external RC target files into the
argument list of autotools macro AC_CONFIG_FILES in configure.ac.

To sum it up:
 * external RC files are now explicitly added to the AC_CONFIG_FILES argument
   list - see configure.ac for more information. Critically the external RC
   files need to be listed before `sac2crc.pre'.
 * external RC files have been renamed to *.in as per autotools specification
 * gitignore file has been updated to ignore generated files now