Adding these options will simplify the FreeBSD port of chibi-scheme
(https://freshports.org/lang/chibi-scheme) because I can get rid of
most of the custom patches currently needed. In FreeBSD pkg-config
files need to be installed into libdata/pkgconfig. INSTALL_EXE
provides a hook for replacing the normal 'install' program with
'install -s' for stripping the binaries/libraries. Adding these
options should have no impact on the default build process.
