check version code is error

pls only check the LIBCURL_VERSION_MAJOR

below is OK
```
#if LIBCURL_VERSION_MAJOR < 7 
#error Required curl 7.40.0+; See http://curl.haxx.se/docs/adv_20150108A.html
#error Supported minimal version is 7.55.0 for header file changes, see Issue #12 (https://github.com/sinofool/build-libcurl-ios/issues/12)
#endif"|gcc -c -o /dev/null -xc -||exit 9
```