build: fix ninja build (#39)

CMake Error:
  Running

   '/usr/bin/ninja' '-C' '/home/thode/external/cfl/CMakeFiles/CMakeTmp' '-t' 'recompact'

  failed with:

   ninja: error: build.ninja:50: bad $-escape (literal $ must be written as $$)
    FLAGS = -Wall -D__FILENAME__='"$(subst /home/thode/external/cfl/,,$(ab...
                                   ^ near here

Signed-off-by: Thomas Devoogdt <[email protected]>

CMakeLists.txt

@@ -42,7 +42,7 @@ endif()
 
 # Define __FILENAME__ consistently across Operating Systems
 if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Windows")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILENAME__='\"$(subst ${CMAKE_SOURCE_DIR}/,,$(abspath $<))\"'")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILENAME__='\"$$(subst ${CMAKE_SOURCE_DIR}/,,$$(abspath $$<))\"'")
 else()
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D__FILENAME__=__FILE__")
 endif()