cmake: Do not cache GFLAGS_NAMESPACE and GFLAGS_INCLUDE_DIR.

These CMake variables are now set to a default which both maintains binary backwards compatibility with previous versions of the library, but at the same time allows already the use of the new "gflags" namespace instead of "google". Users may still override the default using the -D option of cmake, for example, when they use the library directly as submodule in their own project and prefer a different namespace/installation directory.

CMakeLists.txt

@@ -26,7 +26,11 @@ set (PACKAGE_SOVERSION "${PACKAGE_VERSION_MAJOR}")
 
 # ----------------------------------------------------------------------------
 # options
-set (GFLAGS_NAMESPACE "google;${PACKAGE_NAME}" CACHE STRING "C++ namespace identifier(s) of gflags library.")
+if (NOT GFLAGS_NAMESPACE)
+  # maintain binary backwards compatibility with gflags library version <= 2.0,
+  # but at the same time enable the use of the preferred new "gflags" namespace
+  set (GFLAGS_NAMESPACE "google;${PACKAGE_NAME}")
+endif ()
 set (GFLAGS_NAMESPACE_SECONDARY "${GFLAGS_NAMESPACE}")
 list (REMOVE_DUPLICATES GFLAGS_NAMESPACE_SECONDARY)
 if (NOT GFLAGS_NAMESPACE_SECONDARY)
@@ -51,7 +55,6 @@ option (INSTALL_HEADERS            "Request packaging of headers and other devel
 
 mark_as_advanced (CLEAR CMAKE_INSTALL_PREFIX)
 mark_as_advanced (CMAKE_CONFIGURATION_TYPES
-                  GFLAGS_NAMESPACE
                   BUILD_STATIC_LIBS
                   BUILD_NC_TESTS
                   INSTALL_HEADERS)
@@ -72,9 +75,8 @@ if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CXX_FLAGS)
   set_property (CACHE CMAKE_BUILD_TYPE PROPERTY VALUE Release)
 endif ()
 
-set (GFLAGS_INCLUDE_DIR "" CACHE STRING "Installation directory of header files relative to CMAKE_INSTALL_PREFIX.")
 if (NOT GFLAGS_INCLUDE_DIR)
-  set_property (CACHE GFLAGS_INCLUDE_DIR PROPERTY VALUE "${PACKAGE_NAME}")
+  set (GFLAGS_INCLUDE_DIR "${PACKAGE_NAME}")
 else ()
   if (IS_ABSOLUTE GFLAGS_INCLUDE_DIR)
     message (FATAL_ERROR "GFLAGS_INCLUDE_DIR must be a path relative to CMAKE_INSTALL_PREFIX/include")