spandsp: Fix libjpeg checks for --enable-builtin-tiff builds.

spandsp: Fix libjpeg checks for --enable-builtin-tiff builds.

The --enable-builtin-tiff option appends libs/tiff-3.8.2/libtiff/libtiff.la
to LIBS, causing the AC_CHECK_LIB([jpeg]...) check to fail, because
libtiff.la does not exist at configure time.

Temporarily store tiff and jpeg libs in TIFF_-/JPEG_LIBS variables and
append them to LIBS after all library checks have run.

Example error output:
    configure:20049: checking for jpeg_start_compress in -ljpeg
    configure:20074: cc -o conftest -O2 -pipe -fno-strict-aliasing    -L/usr/local/lib conftest.c -ljpeg  -lm  /usr/home/ports/net/freeswitch-core-devel/work/freeswitch-1.2.1/libs/tiff-3.8.2/libtiff/libtiff.la >&5
    cc: /usr/home/ports/net/freeswitch-core-devel/work/freeswitch-1.2.1/libs/tiff-3.8.2/libtiff/libtiff.la: No such file or directory

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>