Adding plugin interface for C files.

Implementation of hooks using C interface basedon dlfcn.  Added test
plugin and makefile to build it.  In order to test it add the followin
into your profrc:
    [plugins]
    load=test-c-plugin.so
and execute profanity piping stderr to some file.  The file should
contain all entries whenever the plugin function is triggered.  It seem
to be workin but some parts are missing.
Minor fix: Reverting back the optimisation level.

FYI: there is no debug version that can be easily obtained.
Removed trailing whitespace