Minor fix for finding libcurl/building sslstrip

- 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.