autotools: Don’t build DLLs on Cygwin by default

By default, OTF does not build DLLs on Cygwin.  Consequently, users who
built OTF and ppamltracer using the default 'configure' flags received
warnings about missing DLLs during ppamltracer’s link phase.  This
commit changes that behavior, causing the generated configure script to
default to only building static libraries in Cygwin.  Should users wish
to build DLLs, they can still do so by passing “--enable-shared” to
configure.