Add configure-time check for std::make_unique

Fixes the build on Clang 6 and closes #4685.

cmake/ConfigureChecks.cmake

@@ -113,4 +113,14 @@ IF(NOT TPARM_TAKES_VARARGS)
   SET(TPARM_SOLARIS_KLUDGE 1)
 ENDIF()
 
+CHECK_CXX_SOURCE_COMPILES("
+#include <memory>
+
+int main () {
+  std::unique_ptr<int> foo = std::make_unique<int>();
+}
+"
+  HAVE_STD__MAKE_UNIQUE
+)
+
 FIND_PROGRAM(SED sed)

configure.ac

@@ -400,6 +400,15 @@ AC_TRY_LINK( [ #include <wchar.h> ],
              [AC_MSG_RESULT(no)],
              )
 
+AC_MSG_CHECKING([for std::make_unique])
+AC_TRY_LINK( [ #include <memory> ],
+             [ std::unique_ptr<int> foo = std::make_unique<int>(); ],
+             [ AC_MSG_RESULT(yes)
+               AC_DEFINE(HAVE_STD__MAKE_UNIQUE, 1, Define to 1 if you have the `std::make_unique' function.)
+             ],
+             [AC_MSG_RESULT(no)],
+             )
+
 if test x$local_gettext != xno; then
   AC_CHECK_FUNCS( gettext )
 

src/common.h

@@ -606,7 +606,7 @@ wcstring vformat_string(const wchar_t *format, va_list va_orig);
 void append_format(wcstring &str, const wchar_t *format, ...);
 void append_formatv(wcstring &str, const wchar_t *format, va_list ap);
 
-#ifdef __cpp_lib_make_unique
+#ifdef HAVE_STD__MAKE_UNIQUE
 using std::make_unique;
 #else
 /// make_unique implementation