Remove commented checks in configure.ac; if we need them again, we can add them.
Change mixed indentations to tabs in configure.ac.
Remove more commented stuff in configure.ac.
Squeeze big if-then-else blocks into AM_CONDITIONAL commands.
Erased useless 'true' in g5proc argument.
Cleanup ETL/m4/ETL.m4
Use AS_HELP_STRING for configure options in synfig-core.