Fix Clang builds on Windows (google#1021)

Fixes google#974. The `cxx_feature_check` now has an additional
optional argument which can be used to supply extra cmake flags
to pass to the `try_compile` command. The `CMAKE_CXX_STANDARD=14`
flag was determined to be the minimum flag necessary to correctly
compile and run the regex feature checks when compiling with Clang
under Windows (n.b. this does *not* refer to clang-cl, the frontend
to the MSVC compiler). The additional flag is not enabled for any
other compiler/platform tuple.

Author: jeremyong

CMakeLists.txt

@@ -245,18 +245,25 @@ if (BENCHMARK_USE_LIBCXX)
   endif()
 endif(BENCHMARK_USE_LIBCXX)
 
+set(EXTRA_CXX_FLAGS "")
+if (WIN32 AND "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+  # Clang on Windows fails to compile the regex feature check under C++11
+  set(EXTRA_CXX_FLAGS "-DCMAKE_CXX_STANDARD=14")
+endif()
+
 # C++ feature checks
 # Determine the correct regular expression engine to use
-cxx_feature_check(STD_REGEX)
-cxx_feature_check(GNU_POSIX_REGEX)
-cxx_feature_check(POSIX_REGEX)
+cxx_feature_check(STD_REGEX ${EXTRA_CXX_FLAGS})
+cxx_feature_check(GNU_POSIX_REGEX ${EXTRA_CXX_FLAGS})
+cxx_feature_check(POSIX_REGEX ${EXTRA_CXX_FLAGS})
 if(NOT HAVE_STD_REGEX AND NOT HAVE_GNU_POSIX_REGEX AND NOT HAVE_POSIX_REGEX)
   message(FATAL_ERROR "Failed to determine the source files for the regular expression backend")
 endif()
 if (NOT BENCHMARK_ENABLE_EXCEPTIONS AND HAVE_STD_REGEX
         AND NOT HAVE_GNU_POSIX_REGEX AND NOT HAVE_POSIX_REGEX)
   message(WARNING "Using std::regex with exceptions disabled is not fully supported")
 endif()
+
 cxx_feature_check(STEADY_CLOCK)
 # Ensure we have pthreads
 set(THREADS_PREFER_PTHREAD_FLAG ON)

cmake/CXXFeatureCheck.cmake

@@ -27,6 +27,11 @@ function(cxx_feature_check FILE)
     return()
   endif()
 
+  if (ARGC GREATER 1)
+    message(STATUS "Enabling additional flags: ${ARGV1}")
+    list(APPEND BENCHMARK_CXX_LINKER_FLAGS ${ARGV1})
+  endif()
+
   if (NOT DEFINED COMPILE_${FEATURE})
     message(STATUS "Performing Test ${FEATURE}")
     if(CMAKE_CROSSCOMPILING)