CMake: resolve undefined PROJECT_VERSION warning for quadriflow/lemon

While it seems harmless, LEMON_VERSION wasn't set by our CMake files.

extern/quadriflow/CMakeLists.txt

@@ -17,6 +17,8 @@ if(WIN32)
 endif()
 
 set(LEMON_3RD_PATH 3rd/lemon-1.3.1)
+# Needed for the configure script.
+set(PROJECT_VERSION "1.3.1")
 
 set(LEMON_SOURCE_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/${LEMON_3RD_PATH})
 
@@ -28,6 +30,8 @@ configure_file(
   ${LEMON_SRC}/config.h.in
   ${LEMON_GEN_DIR}/lemon/config.h
 )
+unset(PROJECT_VERSION)
+
 set(LEMON_SOURCES
   ${LEMON_SRC}/arg_parser.cc
   ${LEMON_SRC}/base.cc