Add LOCAL_OSMOCOM cmake option

When set the gr-gsm will be compiled and linked with the local copies of
the osmocom libraries.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f032206..bfbfe1a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -136,6 +136,8 @@
 find_package(Volk)
 find_package(CppUnit)
 #find_package(Doxygen)
+
+option(LOCAL_OSMOCOM "Build with local osmocom libraries" OFF)
 find_package(Libosmocore)
 find_package(Libosmocodec)
 find_package(Libosmocoding)
@@ -153,7 +155,12 @@
 if(NOT CPPUNIT_FOUND)
     message(FATAL_ERROR "CppUnit required to compile gr-gsm")
 endif()
-if(NOT LIBOSMOCORE_FOUND OR NOT LIBOSMOCODEC_FOUND)
+if(LOCAL_OSMOCOM)
+    message(STATUS "Compiling with local osmocom libraries")
+    unset(LIBOSMOCORE_FOUND)
+    unset(LIBOSMOCODEC_FOUND)
+    unset(LIBOSMOCODING_FOUND)
+elseif(NOT LIBOSMOCORE_FOUND OR NOT LIBOSMOCODEC_FOUND)
     message(STATUS "Compiling local version of libosmocore")
 elseif(NOT LIBOSMOCODING_FOUND)
     message(STATUS "Compiling local version of libosmocoding")