upgrade libevent to 2.1.12

Summary:
libevent 2.1.8 is not aware of the arc4random deprecation on FreeBSD,
so upgrade to the latest.

Reviewed By: xavierd

Differential Revision: D33984680

fbshipit-source-id: b7d2b869bc47d49d8f29df1b318d8f2067204aae

build/fbcode_builder/manifests/libevent

@@ -15,12 +15,12 @@ libevent-devel
 # to use the github generated source tarball rather than
 # the explicitly uploaded source tarball
 [download]
-url = https://github.com/libevent/libevent/archive/release-2.1.8-stable.tar.gz
-sha256 = 316ddb401745ac5d222d7c529ef1eada12f58f6376a66c1118eee803cb70f83d
+url = https://github.com/libevent/libevent/releases/download/release-2.1.12-stable/libevent-2.1.12-stable.tar.gz
+sha256 = 92e6de1be9ec176428fd2367677e61ceffc2ee1cb119035037a27d346b0403bb
 
 [build]
 builder = cmake
-subdir = libevent-release-2.1.8-stable
+subdir = libevent-2.1.12-stable
 
 [cmake.defines]
 EVENT__DISABLE_TESTS = ON
@@ -31,5 +31,8 @@ EVENT__DISABLE_REGRESS = ON
 [cmake.defines.shared_libs=on]
 EVENT__BUILD_SHARED_LIBRARIES = ON
 
-[dependencies.not(os=linux)]
+[cmake.defines.os=windows]
+EVENT__LIBRARY_TYPE = STATIC
+
+[dependencies.not(any(os=linux, os=freebsd))]
 openssl