Summary:
D15683387 introduced manifest for libcurl and used cmake instead of autoconf
(rationale: having cmake build curl would make it easy as we want curl to build
on windows and macos).

if libcurl finds ldap.h, then it builds with ldap enabled.
On my devserver, ldap library is not installed, and thus libcurl builds without
it, and links with FBOSS binaries fine.

In sandbox, ldap seems to be installed, thus libcurl builds with ldap. However,
linking fails for FBOSS binaries as those don't link against ldap - FBOSS does
not depend on ldap.

Thus, we need a way to configure libcurl with ldap disabled. [cmake.defines]
CURL_DISABLE_LDAP=OFF, does not quite do it. Thus, am going with autoconf, with
args --disable-ldap for now for Linux and cmake for non-Linux environments.

Reviewed By: wez

Differential Revision: D15730947

fbshipit-source-id: 2ce48976ee785401d550720cbdccb22f9d416675