cmake: Permit disabling external http-parser

When attempting to build libgit2 as an isolated static lib, CMake
gleefully attempts to use the system http-parser.  This is typically
seen on Linux systems which install header files with every package,
such as Gentoo.

Allow developers to forcibly disable using the system http-parser with
the config switch USE_EXT_HTTP_PARSER.  Defaults to ON to maintain
previous behavior.

Signed-off-by: Jason Cooper <jason@lakedaemon.net>

CMakeLists.txt

@@ -43,6 +43,7 @@ OPTION( USE_SSH				"Link with libssh to enable SSH support" ON )
 OPTION( USE_GSSAPI			"Link with libgssapi for SPNEGO auth"   OFF )
 OPTION( VALGRIND			"Configure build for valgrind"			OFF )
 OPTION( CURL			"Use curl for HTTP if available" ON)
+OPTION( USE_EXT_HTTP_PARSER		"Use system HTTP_Parser if available" ON)
 OPTION( DEBUG_POOL			"Enable debug pool allocator"			OFF )
 
 IF(DEBUG_POOL)
@@ -329,12 +330,12 @@ ENDIF()
 
 # Optional external dependency: http-parser
 FIND_PACKAGE(HTTP_Parser)
-IF (HTTP_PARSER_FOUND AND HTTP_PARSER_VERSION_MAJOR EQUAL 2)
+IF (USE_EXT_HTTP_PARSER AND HTTP_PARSER_FOUND AND HTTP_PARSER_VERSION_MAJOR EQUAL 2)
 	INCLUDE_DIRECTORIES(${HTTP_PARSER_INCLUDE_DIRS})
 	LINK_LIBRARIES(${HTTP_PARSER_LIBRARIES})
 	LIST(APPEND LIBGIT2_PC_LIBS "-lhttp_parser")
 ELSE()
-	MESSAGE(STATUS "http-parser version 2 was not found; using bundled 3rd-party sources.")
+	MESSAGE(STATUS "http-parser version 2 was not found or disabled; using bundled 3rd-party sources.")
 	INCLUDE_DIRECTORIES(deps/http-parser)
 	FILE(GLOB SRC_HTTP deps/http-parser/*.c deps/http-parser/*.h)
 ENDIF()