libgtkada_gl ignore the DESTDIR setting during pacakge installation

[qunying]

The latest release 17.0, after compilation, the command
make DESTDIR=PKG_DIR install

Will install libgtkada_gl part into the system directory in stead of the DESTDIR specified in the command line.

This is because in Makefilie.in

105 ifeq (${HAVE_OPENGL}, True)
            ${GPRINSTALL_FULL} -XLIBRARY_TYPE=$(@F) --build-name=$(@F) \
                  --sources-subdir=include/gtkada/gtkada_gl.$(@F) \
                  --lib-subdir=${libdir}/gtkada/gtkada_gl.$(@F) \
                  --Psrc/opengl/gtkada_gl.gpr
110 endif

The lib-subdir is given an absolute directory without the DESTDIR being consider, it should be the same as other that use
--lib-subdir=lib/gtkada/gtkada_gl.$(@F)
or
-lib-subdir=${DESTDIR}${libdir}/gtkada/gtkada_gl.$(@F)

[setton]

I agree. Would you like to make a pull request for this?

[setton]

Thank you for making the pull request! This is now merged.