CMake: use -O0 in debug mode

Otherwise, GCC optimizes variables away and gdb can't tell us what's
in them.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

CMakeLists.txt

@@ -65,6 +65,7 @@ IF (MSVC)
 	SET(WIN_RC "src/win32/git2.rc")
 ELSE ()
 	SET(CMAKE_C_FLAGS "-O2 -g -Wall -Wextra -Wstrict-aliasing=2 -Wstrict-prototypes -Wmissing-prototypes ${CMAKE_C_FLAGS}")
+	SET(CMAKE_C_FLAGS_DEBUG "-O0 -g")
 	IF (NOT MINGW) # MinGW always does PIC and complains if we tell it to
 		SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")
 	ENDIF ()