Improve CMake exported targets.

I _think_ this represents some of the "best practices" for exporting
targets. They'll be available in a `googletest::` namespace (e.g.
`googletest::gmock`) with non-namespaced `ALIAS` targets.

- Added GOOGLETEST_VERSION variable
- Use `CMakePackageConfigHelpers`, bump minimum CMake version to 2.8.8

Signed-off-by: Dakota Hawkins <dakotahawkins@gmail.com>

cmake/googletestConfig.cmake.in

@@ -0,0 +1,35 @@
+@PACKAGE_INIT@
+
+set(googletest_BUILD_SHARED_LIBS @BUILD_SHARED_LIBS@)
+
+set(googletest_NAMESPACE_TARGETS)
+set(googletest_ALL_INCLUDE_DIRS)
+
+foreach(target @googletest_install_targets@)
+  include(${CMAKE_CURRENT_LIST_DIR}/${target}ConfigInternal.cmake)
+
+  add_library(googletest::${target} INTERFACE IMPORTED)
+  set_target_properties(googletest::${target}
+    PROPERTIES
+      INTERFACE_LINK_LIBRARIES googletest_${target}
+      IMPORTED_GLOBAL ON)
+  if(googletest_BUILD_SHARED_LIBS)
+    set_target_properties(googletest::${target}
+      PROPERTIES
+        INTERFACE_COMPILE_DEFINITIONS "GTEST_LINKED_AS_SHARED_LIBRARY=1")
+  endif()
+  add_library(${target} ALIAS googletest::${target})
+
+  get_target_property(${target}_INCLUDE_DIRS googletest_${target} INTERFACE_INCLUDE_DIRECTORIES)
+
+  list(APPEND googletest_ALL_INCLUDE_DIRS ${${target}_INCLUDE_DIRS})
+  list(APPEND googletest_NAMESPACE_TARGETS googletest::${target})
+endforeach()
+
+list(REMOVE_DUPLICATES googletest_ALL_INCLUDE_DIRS)
+set(GOOGLETEST_INCLUDE_DIRS ${googletest_ALL_INCLUDE_DIRS})
+
+list(REMOVE_DUPLICATES googletest_NAMESPACE_TARGETS)
+set(GOOGLETEST_LIBRARIES ${googletest_NAMESPACE_TARGETS})
+
+set(GOOGLETEST_VERSION "@GOOGLETEST_VERSION@")