Do not unconditionally set CMAKE_C_FLAGS_RELEASE

[danfe]

Most GNU/Linux and *BSD distributions build their packages with certain default compilation flags, e.g. on FreeBSD, our framework sets -DCMAKE_C_FLAGS_RELEASE:STRING="${CFLAGS} -DNDEBUG" and CFLAGS already includes -O2 plus some other useful options, or it could be set by the user to her liking which we should respect. May I suggest the following small patch to CMakeLists.txt:

 # Set common C compiler flags for all targets (the library and the programs).
-set(CMAKE_C_FLAGS_RELEASE "-O2 -DNDEBUG")
+if(NOT CMAKE_C_FLAGS_RELEASE)
+    set(CMAKE_C_FLAGS_RELEASE "-O2 -DNDEBUG")
+endif()
 set(CMAKE_C_STANDARD 99)