Build: fix the build script that looks for yajl

The script was looking for the wrong header file. Instead of look for
yajl_parse.h it was looking for yajl.h. Originally reported by
@rpfilomeno at #804.
Adds curl support to main core

Curl was used only by mlogc, as we want to expand ModSecurity to load
external rules/resources it is now a dependency of the core as well.
Adds support to load remote resources to pmFromFile and ipMatchFromFile

Initially those directives were only able to load content from a
local file. This commit extends this functionality allowing the user to
provide an HTTP URI that can be downloaded and loaded by ModSecurity.
Initially the download is associated with a server restart. For next
versions we expect to load such resources as it become outdated (Without
need to resetart the server).
Initial support to load rules from a remote server

New directive `SecRemoteRules' was added. It allows the user to load a
set of rules from a given HTTP server.
Adds the SecRemoteRulesFailAction directive

This directive allows the user to set a default action whenever the
SecRemoteRules failed to download a set of rules. Current the supported
values are: Warn and Abort. By default `Abort' is selected.
Adds SecRemoteRules as an build option

SecRemoteRules adds a new dependency to libcurl. Before only mlogc was
depending on libcurl. SecRemoteRules also depends on the apr-tools with
crypto support, which (as reported by our buildbots) is not default in
some environments such as: MacOS X. This commit disable SecRemoteRules
support if apr-tools was not compiled with crypto support.
Adds the `crypto' option to SecRemoteRules directive

Originally the SecRemoteRules fetch the rules from an remote server in an
specific format, using cryptography. This patch adds the possibility to
load rules in plain/text format.