Add compile flags and test for compiler options only if we use gcc.

cmake/Modules/DefineCompilerFlags.cmake

@@ -3,6 +3,9 @@
 include(CheckCCompilerFlag)
 
 if (UNIX AND NOT WIN32)
+  if (CMAKE_COMPILER_IS_GNUCC)
+    add_definitions(-Wall -Wextra -Wmissing-prototypes -Wdeclaration-after-statement -Wunused)
+
     # with -fPIC
     check_c_compiler_flag("-fPIC" WITH_FPIC)
     if (WITH_FPIC)
@@ -34,10 +37,6 @@ if (UNIX AND NOT WIN32)
       add_definitions(${_lfs_CFLAGS})
     endif (CMAKE_SIZEOF_VOID_P MATCHES "8")
 
-  if (CMAKE_COMPILER_IS_GNUCC)
-    add_definitions(-Wall -Wextra -Wmissing-prototypes -Wdeclaration-after-statement -Wunused)
-  endif (CMAKE_COMPILER_IS_GNUCC)
-
     check_c_compiler_flag("-fstack-protector" WITH_STACK_PROTECTOR)
     if (WITH_STACK_PROTECTOR)
       add_definitions(-fstack-protector)
@@ -47,5 +46,5 @@ if (UNIX AND NOT WIN32)
     if (WITH_FORTIFY_SOURCE)
       add_definitions(-D_FORTIFY_SOURCE=2)
     endif (WITH_FORTIFY_SOURCE)
-
+  endif (CMAKE_COMPILER_IS_GNUCC)
 endif (UNIX AND NOT WIN32)