Modernize CMakeLists.txt and make gtest download optional

cmake/DownloadGTest.cmake

@@ -7,6 +7,9 @@ configure_file(
                ${CMAKE_BINARY_DIR}/googletest-download/CMakeLists.txt
 )
 
+set(PHMAP_SAVE_CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+set(PHMAP_SAVE_CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_RUNTIME_OUTPUT_DIRECTORY})
+
 # Configure and build the downloaded googletest source
 execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
                 RESULT_VARIABLE result
@@ -24,6 +27,9 @@ if(result)
          message(FATAL_ERROR "Build step for googletest failed: ${result}")
 endif()
 
+set(CMAKE_CXX_FLAGS ${PHMAP_SAVE_CMAKE_CXX_FLAGS})
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${PHMAP_SAVE_CMAKE_RUNTIME_OUTPUT_DIRECTORY})
+
 # Prevent overriding the parent project's compiler/linker settings on Windows
 set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
 

cmake/helpers.cmake

@@ -15,8 +15,7 @@ function(phmap_cc_test)
   )
 
   set(_NAME "test_${PHMAP_CC_TEST_NAME}")
-  add_executable(${_NAME} "")
-  target_sources(${_NAME} PRIVATE ${PHMAP_CC_TEST_SRCS})
+  add_executable(${_NAME}  ${PHMAP_CC_TEST_SRCS})
   target_include_directories(${_NAME}
     PUBLIC ${PHMAP_COMMON_INCLUDE_DIRS}
     PRIVATE ${GMOCK_INCLUDE_DIRS} ${GTEST_INCLUDE_DIRS}