Declare WITH_DEBUG_MESSAGES as a cmake option

When it is not declared as an option, the SetCompilerFlags.cmake scripts
test only its definition and not its value. Hence there was no diffence between :
./gbuild.sh -d ON   and    ./gbuild.sh -d OFF  . Both cases lead to activate the
debug messages such as DEBUG_LP

Author: benlabbe

SetCompilerFlags.cmake

@@ -22,6 +22,8 @@ option(WITH_ASAN "Enable address sanitizer" OFF)
 message("WITH_ASAN=${WITH_ASAN}")
 option(WITH_ARCH "Enable architecture optimizations" OFF)
 message("WITH_ARCH=${WITH_ARCH}")
+option(WITH_DEBUG_MESSAGES "Enable debug messages" OFF)
+message("WITH_DEBUG_MESSAGES=${WITH_DEBUG_MESSAGES}")
 
 
 if (${CMAKE_SYSTEM_NAME} STREQUAL "Linux")