hivex: Added gnulib includes from builddir, as suggested by the Gnulib documentation...

hivex: Added gnulib includes from builddir, as suggested by the Gnulib documentation; link hivexml against libgnu.

    Since some modules (`getopt', for example) may copy files
    into the build directory, `top_builddir/lib' is needed as well as
    `top_srcdir/lib'. -- GNU Gnulib manual, section 2.2 Initial import

This fixes an in-tree build failure on a Debian/sid system (see
below). hivexml could be built out-of-tree, but it turned out that due
to a missing include path, in this case the system's getopt
implementation was used insted of Gnulib's.

make[2]: Entering directory `«SRCDIR»/xml'
gcc -std=gnu99 -DHAVE_CONFIG_H -I. -I..    -DLOCALEBASEDIR=\""/usr/local/share/locale"\" -I../gnulib/lib -I../lib -I/usr/include/libxml2     -g -O2 -MT hivexml-hivexml.o -MD -MP -MF .deps/hivexml-hivexml.Tpo -c -o hivexml-hivexml.o `test -f 'hivexml.c' || echo './'`hivexml.c
mv -f .deps/hivexml-hivexml.Tpo .deps/hivexml-hivexml.Po
/bin/bash ../libtool  --tag=CC   --mode=link gcc -std=gnu99 -DLOCALEBASEDIR=\""/usr/local/share/locale"\" -I../gnulib/lib -I../lib -I/usr/include/libxml2     -g -O2   -o hivexml hivexml-hivexml.o ../lib/libhivex.la -lxml2
libtool: link: gcc -std=gnu99 -DLOCALEBASEDIR=\"/usr/local/share/locale\" -I../gnulib/lib -I../lib -I/usr/include/libxml2 -g -O2 -o .libs/hivexml hivexml-hivexml.o  ../lib/.libs/libhivex.so /usr/lib/libxml2.so
hivexml-hivexml.o: In function `main':
«SRCDIR»/xml/hivexml.c:96: undefined reference to `rpl_getopt'
«SRCDIR»/xml/hivexml.c:110: undefined reference to `rpl_optind'
«SRCDIR»/xml/hivexml.c:154: undefined reference to `rpl_optind'
collect2: ld returned 1 exit status
make[2]: *** [hivexml] Error 1
make[2]: Leaving directory `«SRCDIR»/xml'