* configure.in: use the sinclude bit again, since it works in
automake 1.4. Generate the Makefile in the embed dir too

* app/gimp.sym: new file, list the symbols we want export to
modules here

* app/Makefile.am: add -export-symbols to LDFLAGS

* plug-ins/Makefile.am: use make variable expansion instead of
direct substitution for optional plug-in dirs. E.g., now one can
do "make GIMP_PERL=" to skip the perl plug-in dir during build
without rerunning configure

* plug-ins/rcm/Makefile.am: add header files to SOURCES

-Yosh