fix: GFLAGS_* variables take precedence in gflags_define

cmake/utils.cmake

@@ -65,16 +65,18 @@ macro (gflags_define type varname docstring default)
     message (FATAL_ERROR "gflags_variable: Too many macro arguments")
   endif ()
   if (NOT DEFINED GFLAGS_${varname})
+    if (DEFINED ${varname})
+      set(GFLAGS_${varname} "${${varname}}")
+    else ()
       if (GFLAGS_IS_SUBPROJECT AND "${ARGC}" EQUAL 5)
         set (GFLAGS_${varname} "${ARGV4}")
       else ()
         set (GFLAGS_${varname} "${default}")
       endif ()
     endif ()
+  endif ()
   if (GFLAGS_IS_SUBPROJECT)
-    if (NOT DEFINED ${varname})
     set (${varname} "${GFLAGS_${varname}}")
-    endif ()
   else ()
     set (${varname} "${GFLAGS_${varname}}" CACHE ${type} "${docstring}")
   endif ()