gdal-lib: fix CONFIGURE_ARGS reference to non-existent directory

The configure script for gdal-lib specifies locations for various
required libraries via command line arguments.  For example, the
configure script defines compiler flags for the location of libz based
upon the --with-libz argument as follows:

EXTRA_INCLUDES="-I$with_libz -I$with_libz/include $EXTRA_INCLUDES"

Since the origin of the package, the location of libz has been
specified in Makefile.common as follows:

CONFIGURE_ARGS+=	--with-libz=${BUILDLINK_PREFIX.zlib}/include

In a typical pkgsrc installation, the configure script is run with the
argument --with-libz=/usr/pkg/include, which leads to the following
compiler flags:

	  -I/usr/pkg/include -I/usr/pkg/include/include

The latter directory of course generally never exists.  Although this
has apparently worked in the past, gcc12 exits with an error.

This patch removes the extra /include suffix from the libz location
specified in CONFIGURE_ARGS.  As a result, libz now matches the way
other libraries are specified, none of which include the /include
suffix and all of which are handled in the configure script in similar
ways.


(brook)