How to build the codec shared libraries
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CODEC_ALAW:
  gcc -fPIC -g -c -Wall ulaw.cpp         
-------> gives you the object file ulaw.o. 
-------> option -g to include debug information
-------> option -Wall to generate warnings
-------> option -fPIC to enable position independant code generation

  gcc -shared -o libcodec_ulaw.so ulaw.o -lc
-------> creates the shared library

CODEC ALAW: 
  idem

CODEC_GSM:
  gcc -fPIC -g -c -Wall gsmcodec.cpp
  gcc -shared -o -libcodec_gsm.so gsmcodec.o -lc -lgsm
-------> You need the standard library libgsm1 installed (with dev package).

REFERENCES: http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

The shared library should be installed in $(libdir)/sflphone/codecs where libdir=$(PREFIX)/lib.
