Allow C++ standard to be passed to CHECK_CXX_SOURCE_COMPILES

Work around the issue in CMake where C++ standard doesn't get propagated
to CHECK_CXX_SOURCE_COMPILES. Also correctly check for std::make_unique;
the define was missing from the config.h header.

cmake/ConfigureChecks.cmake

@@ -4,6 +4,12 @@
 # `wcstod_l` is a GNU-extension, sometimes hidden behind GNU-related defines.
 # This is the case for at least Cygwin and Newlib.
 LIST(APPEND CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE=1)
+
+
+# Work around the fact that cmake does not propagate the language standard flag into
+# the CHECK_CXX_SOURCE_COMPILES function. See CMake issue #16456.
+LIST(APPEND CMAKE_REQUIRED_FLAGS "${CMAKE_CXX${CMAKE_CXX_STANDARD}_STANDARD_COMPILE_OPTION}")
+
 IF(APPLE)
     INCLUDE(CheckCXXCompilerFlag)
     CHECK_CXX_COMPILER_FLAG("-Werror=unguarded-availability" REQUIRES_UNGUARDED_AVAILABILITY)

config_cmake.h.in

@@ -127,6 +127,9 @@
 /* Define to 1 if the _nl_msg_cat_cntr symbol is exported. */
 #cmakedefine HAVE__NL_MSG_CAT_CNTR 1
 
+/* Define to 1 if std::make_unique is available. */
+#cmakedefine HAVE_STD__MAKE_UNIQUE 1
+
 /* Define to 1 if the _sys_errs array is available. */
 #cmakedefine HAVE__SYS__ERRS 1