cython bindings check before proceeding

Compiling libimobiledevice fails with libplists cython flag off (gentoo) with the following error.
... ... ...

  CC     mobile_image_mounter.lo
  CC     screenshotr.lo
  CC     mobilesync.lo
  CC     mobilebackup.lo
  CC     house_arrest.lo
  CC     mobilebackup2.lo
  CC     restore.lo
  CCLD   libimobiledevice.la
make[2]: Leaving directory `/home/godfath3r/test/libimobiledevice-1.1.2/src'
Making all in include
make[2]: Entering directory `/home/godfath3r/test/libimobiledevice-1.1.2/include'
make[2]: Nothing to be done for `all'.
make[2]: Leaving directory `/home/godfath3r/test/libimobiledevice-1.1.2/include'
Making all in cython
make[2]: Entering directory `/home/godfath3r/test/libimobiledevice-1.1.2/cython'
make[2]: *** No rule to make target `/usr/include/plist/cython/plist.pxd', needed by `imobiledevice.c'.  Stop.
make[2]: Leaving directory `/home/godfath3r/test/libimobiledevice-1.1.2/cython'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/godfath3r/test/libimobiledevice-1.1.2'
make: *** [all] Error 2

libimobiledevice should check if cython support is on first. Maybe at ./configure time...