fixed linking with leptonica 1.69

I had problems like #25, the linker missed symbols both from `libgif` and `libwebp` producing output like the following. This commit adds both libraries to the linker, so `jbig2` gets compiled.

	g++ -o jbig2 jbig2.cc -L. -ljbig2enc /usr/lib/liblept.a -I/usr/include/leptonica/ -Wall -I/usr/include -L/usr/lib -O3  -lpng -ljpeg -ltiff -lm -lz
	/usr/lib/liblept.a(webpio.o): In function `pixReadStreamWebP':
	(.text+0x42): undefined reference to `WebPGetInfo'
	/usr/lib/liblept.a(webpio.o): In function `pixReadStreamWebP':
	(.text+0x91): undefined reference to `WebPDecodeRGBAInto'
	/usr/lib/liblept.a(webpio.o): In function `readHeaderWebP':
	(.text+0x1ef): undefined reference to `WebPGetInfo'
	/usr/lib/liblept.a(webpio.o): In function `pixWriteStreamWebP':
	(.text+0x367): undefined reference to `WebPEncodeRGBA'
	/usr/lib/liblept.a(gifio.o): In function `pixReadStreamGif':
	(.text+0x2b): undefined reference to `DGifOpenFileHandle'
	/usr/lib/liblept.a(gifio.o): In function `pixReadStreamGif':
	(.text+0x43): undefined reference to `DGifSlurp'
	/usr/lib/liblept.a(gifio.o): In function `pixReadStreamGif':
	(.text+0x1df): undefined reference to `DGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixReadStreamGif':
	(.text+0x1ee): undefined reference to `DGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixReadStreamGif':
	(.text+0x4a5): undefined reference to `DGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixReadStreamGif':
	(.text+0x4f5): undefined reference to `DGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixReadStreamGif':
	(.text+0x51a): undefined reference to `DGifCloseFile'
	/usr/lib/liblept.a(gifio.o):(.text+0x547): more undefined references to `DGifCloseFile' follow
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x711): undefined reference to `MakeMapObject'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x7a5): undefined reference to `EGifOpenFileHandle'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x7e0): undefined reference to `EGifPutScreenDesc'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x7f7): undefined reference to `FreeMapObject'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x7ff): undefined reference to `EGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x83e): undefined reference to `FreeMapObject'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x86c): undefined reference to `FreeMapObject'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x88c): undefined reference to `EGifPutImageDesc'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0x8a7): undefined reference to `EGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0xa57): undefined reference to `EGifPutLine'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0xa8c): undefined reference to `EGifPutComment'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0xaba): undefined reference to `EGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0xb1d): undefined reference to `FreeMapObject'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0xb73): undefined reference to `EGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0xbe2): undefined reference to `EGifCloseFile'
	/usr/lib/liblept.a(gifio.o): In function `pixWriteStreamGif':
	(.text+0xc0d): undefined reference to `EGifCloseFile'
	collect2: ld returned 1 exit status
	make: *** [jbig2] Error 1