cmake: setup required libraries before checking function

As per the docs[1], another set of variables will be provided by PKG_CHECK_MODULES when static linking (with the additional libraries required for static links). We need to pass those libraries to CHECK_LIBRARY_EXISTS so *its* link succeeds.

1 - https://cmake.org/cmake/help/v3.1/module/FindPkgConfig.html#command:pkg_check_modules

Ref: libgit2/objective-git#645

src/CMakeLists.txt

@@ -292,7 +292,14 @@ IF (LIBSSH2_FOUND)
 	LIST(APPEND LIBGIT2_PC_LIBS ${LIBSSH2_LDFLAGS})
 	#SET(LIBGIT2_PC_LIBS "${LIBGIT2_PC_LIBS} ${LIBSSH2_LDFLAGS}")
 
-	CHECK_LIBRARY_EXISTS("${LIBSSH2_LIBRARIES}" libssh2_userauth_publickey_frommemory "${LIBSSH2_LIBRARY_DIRS}" HAVE_LIBSSH2_MEMORY_CREDENTIALS)
+	# We might need to provide additional libraries so libssh2 links
+	IF(BUILD_SHARED_LIBS)
+		CHECK_LIBRARY_EXISTS(ssh2 libssh2_userauth_publickey_frommemory "${LIBSSH2_LIBRARY_DIRS}" HAVE_LIBSSH2_MEMORY_CREDENTIALS)
+	ELSE()
+		SET(CMAKE_REQUIRED_LIBRARIES ${LIBSSH2_STATIC_LIBRARIES})
+		CHECK_LIBRARY_EXISTS(ssh2 libssh2_userauth_publickey_frommemory "${LIBSSH2_TMP_LIBRARY_DIRS}" HAVE_LIBSSH2_MEMORY_CREDENTIALS)
+		UNSET(CMAKE_REQUIRED_LIBRARIES)
+	ENDIF()
 	IF (HAVE_LIBSSH2_MEMORY_CREDENTIALS)
 		SET(GIT_SSH_MEMORY_CREDENTIALS 1)
 	ENDIF()