Cmake: handle the case of local libgit2 but pkg_config for libssh2

Yes, it's kinda weird but that's what I happen to have on one of my
systems and this seems easy enough to implement.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

CMakeLists.txt

@@ -59,7 +59,10 @@ else()
 	if(USE_LIBGIT23_API)
 		set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DUSE_LIBGIT23_API")
 		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DUSE_LIBGIT23_API")
-		find_package(libssh2 REQUIRED)
+		find_package(libssh2 QUIET)
+		if(!LIBSSH2_FOUND)
+			pkg_config_library(LIBSSH2 libssh2 REQUIRED)
+		endif()
 		set(LIBGIT2_LIBRARIES ${LIBGIT2_LIBRARIES} ${LIBSSH2_LIBRARIES})
 	endif()
 endif()