Fix #538 - gtest.h not found when building with older CMake versions.

Older CMake versions, in particular 2.8, don't seem to correctly handle
interface include directories. This causes failures when building the
tests. Additionally, older CMake versions use a different library install
directory than expected (i.e. they use lib/<target-triple>). This caused
certain tests to fail to link.

This patch fixes both those issues. The first by manually adding the
correct include directory when building the tests. The second by specifying
the library output directory when configuring the GTest build.

cmake/HandleGTest.cmake

@@ -31,8 +31,14 @@ macro(build_external_gtest)
     list(APPEND GTEST_FLAGS "-Wno-unused-function")
   endif()
   split_list(GTEST_FLAGS)
+  set(EXCLUDE_FROM_ALL_OPT "")
+  set(EXCLUDE_FROM_ALL_VALUE "")
+  if (${CMAKE_VERSION} VERSION_GREATER "3.0.99")
+      set(EXCLUDE_FROM_ALL_OPT "EXCLUDE_FROM_ALL")
+      set(EXCLUDE_FROM_ALL_VALUE "ON")
+  endif()
   ExternalProject_Add(googletest
-      EXCLUDE_FROM_ALL ON
+      ${EXCLUDE_FROM_ALL_OPT} ${EXCLUDE_FROM_ALL_VALUE}
       GIT_REPOSITORY https://github.com/google/googletest.git
       GIT_TAG master
       PREFIX "${CMAKE_BINARY_DIR}/googletest"
@@ -42,6 +48,7 @@ macro(build_external_gtest)
         -DCMAKE_C_COMPILER:STRING=${CMAKE_C_COMPILER}
         -DCMAKE_CXX_COMPILER:STRING=${CMAKE_CXX_COMPILER}
         -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
+        -DCMAKE_INSTALL_LIBDIR:PATH=<INSTALL_DIR>/lib
         -DCMAKE_CXX_FLAGS:STRING=${GTEST_FLAGS}
         -Dgtest_force_shared_crt:BOOL=ON
       )
@@ -69,7 +76,7 @@ macro(build_external_gtest)
   add_dependencies(gtest googletest)
   add_dependencies(gtest_main googletest)
   set(GTEST_BOTH_LIBRARIES gtest gtest_main)
-  #set(GTEST_INCLUDE_DIRS ${install_dir}/include)
+  set(GTEST_INCLUDE_DIRS ${install_dir}/include)
 endmacro(build_external_gtest)
 
 if (BENCHMARK_ENABLE_GTEST_TESTS)

test/CMakeLists.txt

@@ -144,8 +144,11 @@ if (BENCHMARK_ENABLE_GTEST_TESTS)
     if (TARGET googletest)
       add_dependencies(${name} googletest)
     endif()
+    if (GTEST_INCLUDE_DIRS)
+      target_include_directories(${name} PRIVATE ${GTEST_INCLUDE_DIRS})
+    endif()
     target_link_libraries(${name} benchmark
-        "${GTEST_BOTH_LIBRARIES}" ${CMAKE_THREAD_LIBS_INIT})
+        ${GTEST_BOTH_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT})
   endmacro(compile_gtest)
 
   macro(add_gtest name)