Minor fix for finding libcurl/building sslstrip

OSX mountain lion has an old version of libcurl, and the build scripts didn't make it easy to point at a more recent build in a different directory on my system. CMake's find_package has several advantages over find_library/find_path. 

If @eaescob (or someone) wants to take a peek at this for me, that'd be great. I'm inclined to pursue some cleaning up of the cmake stuff a bit more, just to make things consistent. 

- Only look for CURL if we can build plugins (HAVE_PLUGINS).

- Use find_package to locate CURL headers *and* libs. This makes it
  possible to specify alternate CURL installation root via
  CMAKE_PREFIX_PATH. (ex: -DCMAKE_PREFIX_PATH=/usr/local/opt/curl/)

- Have cmake only build sslstrip if requirements for CURL and PCRE are
  met.