Handle --disable-plugins* correctly in configure.ac

Oh boy, please double check this! Should fix issue #36.

I checked the CPP_FLAGS in the generated Makefile and it _looks_ like it does what it should, but I'm absolutely unsure about this. As I said in the issue, I don't have any experience with automake and this is solely based on [documentation reading](https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Package-Options.html) and try-and-error.