Introduce option to select default gtest installation. Refs #309

This will introduce RAPIDJSON_BUILD_THIRDPARTY_GTEST option. If it is set to
TRUE, cmake will look for GTest installation in `thirdparty/gtest` before
looking in other places.

Current default value (OFF) for RAPIDJSON_BUILD_THIRDPARTY_GTEST represents
previous behaviour when system-wide gtest installation is used whenever
possible.

This commit will as well eliminate problem described in #309 when source
directory found is `thirdparty/gtest` while include files are found
system-wide. This however won't give the user possibility to select gtest
installation to use.

CMakeLists.txt

@@ -17,6 +17,8 @@ SET(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 option(RAPIDJSON_BUILD_DOC "Build rapidjson documentation." ON)
 option(RAPIDJSON_BUILD_EXAMPLES "Build rapidjson examples." ON)
 option(RAPIDJSON_BUILD_TESTS "Build rapidjson perftests and unittests." ON)
+option(RAPIDJSON_BUILD_THIRDPARTY_GTEST 
+    "Use gtest installation in `thirdparty/gtest` by default if available" OFF)
 
 option(RAPIDJSON_HAS_STDSTRING "" OFF)
 if(RAPIDJSON_HAS_STDSTRING)

CMakeModules/FindGTestSrc.cmake

+
 SET(GTEST_SEARCH_PATH 
     "${GTEST_SOURCE_DIR}"
     "${CMAKE_SOURCE_DIR}/thirdparty/gtest")
 
 IF(UNIX)
-    LIST(INSERT GTEST_SEARCH_PATH 1 "/usr/src/gtest")
+    IF(RAPIDJSON_BUILD_THIRDPARTY_GTEST)
+        LIST(APPEND GTEST_SEARCH_PATH "/usr/src/gtest")
+    ELSE()
+        LIST(INSERT GTEST_SEARCH_PATH 1 "/usr/src/gtest")
+    ENDIF()
 ENDIF()
 
 FIND_PATH(GTEST_SOURCE_DIR
@@ -15,6 +20,7 @@ FIND_PATH(GTEST_SOURCE_DIR
 FIND_PATH(GTEST_INCLUDE_DIR
     NAMES gtest/gtest.h
     PATH_SUFFIXES include
+    HINTS ${GTEST_SOURCE_DIR}
     PATHS ${GTEST_SEARCH_PATH})
 
 INCLUDE(FindPackageHandleStandardArgs)