core: move the "-no-undefined" from LDFLAGS in configure.ac to Makefile.am in plugins

This "-no-undefined" was causing an error when detecting "ld" lib on Cygwin.
core: add detection of pkg-config (required on Cygwin)

This fixes the detection of enchant lib, the macro PKG_CHECK_MODULES requires
pkg-config.
core: fix detection of python on Cygwin (autotools)