The find_curl macro is also checking whether libcurl is linked against
gnutls. However the check depends on "CURL_LIBS" which wasn't defined
by the macro.
This commit will define "CURL_LIBS" so that the check works as expected.
As reported by Rainer Jung, Curl may not be mandatory to build
ModSecurity core. This patch make it optional by:
- Concentrate all downloads using curl on msc_remote_rules.c
- Split Curl build definitions checks into: WITH_CURL, WITH_REMOTE_RULES
and WITH_CRYPTO.
- WITH_CURL: Contains Culr headers and binaries during the build time.
- WITH_REMOTE_RULES: Currently enabled if Curl is present.
- WITH_CRYPTO: Set if apr tool was compiled with crypto support.
- Renames msc_remote_grab_content to msc_remote_download_content
Seems like curl versions older than 7.34.0 does not have support for
`CURL_SSLVERSION_TLSv1_2'. In this cases, using CURL_SSLVERSION_TLSv1
which was added at version 7.9.2. ModSecurity demands a curl version
newer than 7.15.1.
