ac_cxx_explicit_instantiations

If the C++ compiler supports explicit instanciations syntax, define HAVE_INSTANTIATIONS.

AC_DEFUN([AC_CXX_EXPLICIT_INSTANTIATIONS],
[AC_CACHE_CHECK(whether the compiler supports explicit instantiations,
ac_cv_cxx_explinst,
[AC_LANG_SAVE
 AC_LANG_CPLUSPLUS
 AC_TRY_COMPILE([template <class T> class A { T t; }; template class A<int>;],
 [], ac_cv_cxx_explinst=yes, ac_cv_cxx_explinst=no)
 AC_LANG_RESTORE
])
if test "$ac_cv_cxx_explinst" = yes; then
  AC_DEFINE(HAVE_INSTANTIATIONS,,
            [define if the compiler supports explicit instantiations])
fi
])