cmake: Add advanced cache entries for GFLAGS_NAMESPACE and GFLAGS_INCLUDE_DIR

CMakeLists.txt

@@ -29,7 +29,8 @@ set (PACKAGE_SOVERSION "${PACKAGE_VERSION_MAJOR}")
 if (NOT GFLAGS_NAMESPACE)
   # maintain binary backwards compatibility with gflags library version <= 2.0,
   # but at the same time enable the use of the preferred new "gflags" namespace
-  set (GFLAGS_NAMESPACE "google;${PACKAGE_NAME}")
+  set (GFLAGS_NAMESPACE "google;${PACKAGE_NAME}" CACHE STRING "Name(s) of library namespace (separate multiple options by semicolon)")
+  mark_as_advanced (GFLAGS_NAMESPACE)
 endif ()
 set (GFLAGS_NAMESPACE_SECONDARY "${GFLAGS_NAMESPACE}")
 list (REMOVE_DUPLICATES GFLAGS_NAMESPACE_SECONDARY)
@@ -76,7 +77,8 @@ if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CXX_FLAGS)
 endif ()
 
 if (NOT GFLAGS_INCLUDE_DIR)
-  set (GFLAGS_INCLUDE_DIR "${PACKAGE_NAME}")
+  set (GFLAGS_INCLUDE_DIR "${PACKAGE_NAME}" CACHE STRING "Name of include directory of installed header files")
+  mark_as_advanced (GFLAGS_INCLUDE_DIR)
 else ()
   if (IS_ABSOLUTE GFLAGS_INCLUDE_DIR)
     message (FATAL_ERROR "GFLAGS_INCLUDE_DIR must be a path relative to CMAKE_INSTALL_PREFIX/include")