CMake: Fix EXTRA_CFLAGS after commit cae0c65. [skip appveyor]

Same as in libpcap.

(cherry picked from commit bbeedf4f477ff85a0563ae514517798e1022c3ff)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8dec72b..95da37b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -925,7 +925,7 @@
 #
 if(NOT "${EXTRA_CFLAGS}" STREQUAL "")
     foreach(_extra_cflag ${EXTRA_CFLAGS})
-        check_and_add_compiler_option(_extra_cflag)
+        check_and_add_compiler_option("${_extra_cflag}")
     endforeach(_extra_cflag)
     message(STATUS "Added extra compile options (${EXTRA_CFLAGS})")
 endif()