Make it possible to give a configure flag to where deepspeech libs are

[ftyers]

I get:

fran@matxine:~/source/gst-deepspeech$ make
make  all-recursive
make[1]: Entering directory '/home/fran/source/gst-deepspeech'
Making all in src
make[2]: Entering directory '/home/fran/source/gst-deepspeech/src'
/bin/sh ../libtool  --tag=CXX --tag=disable-static  --mode=compile g++ -DHAVE_CONFIG_H -I. -I..    -pthread -I/usr/include/gstreamer-1.0 -I/usr/include/orc-0.4 -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -Wall -g -O2 -MT libgstdeepspeech_la-gstdeepspeech.lo -MD -MP -MF .deps/libgstdeepspeech_la-gstdeepspeech.Tpo -c -o libgstdeepspeech_la-gstdeepspeech.lo `test -f 'gstdeepspeech.cc' || echo './'`gstdeepspeech.cc
libtool: compile:  g++ -DHAVE_CONFIG_H -I. -I.. -pthread -I/usr/include/gstreamer-1.0 -I/usr/include/orc-0.4 -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -Wall -g -O2 -MT libgstdeepspeech_la-gstdeepspeech.lo -MD -MP -MF .deps/libgstdeepspeech_la-gstdeepspeech.Tpo -c gstdeepspeech.cc  -fPIC -DPIC -o .libs/libgstdeepspeech_la-gstdeepspeech.o
gstdeepspeech.cc:63:10: fatal error: deepspeech.h: El fitxer o directori no existeix
 #include <deepspeech.h>
          ^~~~~~~~~~~~~~
compilation terminated.
Makefile:464: recipe for target 'libgstdeepspeech_la-gstdeepspeech.lo' failed
make[2]: *** [libgstdeepspeech_la-gstdeepspeech.lo] Error 1
make[2]: Leaving directory '/home/fran/source/gst-deepspeech/src'
Makefile:406: recipe for target 'all-recursive' failed
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory '/home/fran/source/gst-deepspeech'
Makefile:338: recipe for target 'all' failed
make: *** [all] Error 2

when I try to compile. Because my deepspeech.h is in ../DeepSpeech/native_client/

[crypdick]

See http://blog.mikeasoft.com/2017/12/30/speech-recognition-mozillas-deepspeech-gstreamer-and-ibus/#comment-1120291

Deepspeech native client: https://github.com/mozilla/DeepSpeech/tree/master/native_client