CMake: make it possible to pass ABC_USE_NAMESPACE=FALSE, instead of just empty ABC_USE_NAMESPACE

CMakeLists.txt

@@ -44,7 +44,7 @@ elseif(READLINE_FOUND MATCHES FALSE)
 endif()
 
 if(ABC_USE_NAMESPACE)
-    set(ABC_USE_NAMESPACE "ABC_USE_NAMESPACE=${ABC_USE_NAMESPACE}")
+    set(ABC_USE_NAMESPACE_FLAGS "ABC_USE_NAMESPACE=${ABC_USE_NAMESPACE}")
 endif()
 
 # run make to extract compiler options, linker options and list of source files
@@ -52,7 +52,7 @@ execute_process(
   COMMAND
     make
         ${ABC_READLINE_FLAGS}
-        ${ABC_USE_NAMESPACE}
+        ${ABC_USE_NAMESPACE_FLAGS}
         ARCHFLAGS_EXE=${CMAKE_CURRENT_BINARY_DIR}/abc_arch_flags_program.exe
         ABC_MAKE_NO_DEPS=1
         CC=${CMAKE_C_COMPILER}