Change default GFLAGS_NAMESPACE to "google;gflags" and add GFLAGS_INCLUDE_DIR…

Change default GFLAGS_NAMESPACE to "google;gflags" and add GFLAGS_INCLUDE_DIR with default "gflags" to cache.

This ensures binary compatibility with previous 2.x release versions while allowing users to refer to the gflags library symbols using the preferred "gflags" namespace identifier.

CMakeLists.txt

@@ -26,7 +26,7 @@ set (PACKAGE_SOVERSION "${PACKAGE_VERSION_MAJOR}")
 
 # ----------------------------------------------------------------------------
 # options
-set (GFLAGS_NAMESPACE "${PACKAGE_NAME};google" CACHE STRING "C++ namespace identifier(s) of gflags library.")
+set (GFLAGS_NAMESPACE "google;${PACKAGE_NAME}" CACHE STRING "C++ namespace identifier(s) of gflags library.")
 set (GFLAGS_NAMESPACE_SECONDARY "${GFLAGS_NAMESPACE}")
 list (REMOVE_DUPLICATES GFLAGS_NAMESPACE_SECONDARY)
 if (NOT GFLAGS_NAMESPACE_SECONDARY)
@@ -72,8 +72,9 @@ 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 (GFLAGS_INCLUDE_DIR "${GFLAGS_NAMESPACE}")
+  set_property (CACHE GFLAGS_INCLUDE_DIR PROPERTY VALUE "${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")