synfig-core/configure.ac: Squeeze some if-else blocks into equivalent AM_CONDITIONAL blocks.
synfig-core/configure.ac: Replace manual help strings with AS_HELP_STRING.
synfig-core/configure.ac: Take out if-else block that I forgot.
synfig-core/configure.ac: Shorten help strings.
synfig-core/configure.ac: Erase commented-out code.  They can be rewritten if we need them.
synfig-core/configure.ac: Erase lonely 'true' that did nothing.
synfig-core/configure.ac: Turn mixed indentation (tabs+spaces) into just tabs.
synfig-core/configure.ac: Erase more commented-out code.