configure: check for introspection macro before calling it

In case the person building nautilus didn't have introspection,
autoconf was not failing when finding the call to
GOBJECT_INTROSPECTION_CHECK. After, automake would
throw this kind of cryptic message (which hints about
introspection not being present, but also smells like a
bug in the autotools scripts waiting to be fixed):

...
eel/Makefile.am: installing './depcomp'
parallel-tests: installing './test-driver'
libnautilus-extension/Makefile.am:67: error: HAVE_INTROSPECTION does not appear in AM_CONDITIONAL

We can improve this by checking if the macro is defined before
calling it, which would then not pose any problem in case the
user wants to build without introspection support.

https://bugzilla.gnome.org/show_bug.cgi?id=702554