[#2021] Fix a mistake in the readme from sflphone-common that gives wrong

compilation information for pjsip-1.0.3.

sflphone-common/README

@@ -67,9 +67,19 @@ How to compile
 # First compile the SIP library
 cd libs/pjproject-1.0.3
 ./autogen.sh
-make dep && make
+./configure --disable-ilbc-codec && make clean && make dep && make
 sudo make install
 
+If you already have installed a different version of 
+pjsip on your system, you will most likely have some errors
+such as :
+
+"ln: creating symbolic link `./libpjsip-sfl.a': File exists"
+
+after executing make install.
+
+Simply remove those files and run "make install" again.
+
 # Then the daemon
 cd ../..
 ./autogen.sh --prefix=/usr --with-debug