Let environment CFLAGS override the debug flags

CMakeLists.txt

@@ -93,8 +93,8 @@ IF (MSVC)
 
    # Precompiled headers
 ELSE ()
+	SET(CMAKE_C_FLAGS_DEBUG "-O0 -g ${CMAKE_C_FLAGS}")
 	SET(CMAKE_C_FLAGS "-O2 -g -D_GNU_SOURCE -Wall -Wextra -Wno-missing-field-initializers -Wstrict-aliasing=2 -Wstrict-prototypes -Wmissing-prototypes ${CMAKE_C_FLAGS}")
-	SET(CMAKE_C_FLAGS_DEBUG "-O0 -g")
 	IF (MINGW) # MinGW always does PIC and complains if we tell it to
 		STRING(REGEX REPLACE "-fPIC" "" CMAKE_SHARED_LIBRARY_C_FLAGS "${CMAKE_SHARED_LIBRARY_C_FLAGS}")
 	ELSE ()