explicitly switch to C++-tests

[[Imported from SVN: r959]]

m4/dune_all.m4

@@ -8,6 +8,7 @@
 # dune-directory
 
 AC_DEFUN([DUNE_CHECK_ALL],[
+  AC_LANG_PUSH([C++])
 dnl check for programs
   AC_REQUIRE([AC_PROG_CXX])
   AC_REQUIRE([AC_PROG_CPP])
@@ -60,6 +61,7 @@ dnl check all components
   AC_SUBST(ALL_PKG_LIBS, "$LIBS $DUNE_PKG_LIBS")
   AC_SUBST(ALL_PKG_LDFLAGS, "$LDFLAGS $DUNE_PKG_LDFLAGS")
   AC_SUBST(ALL_PKG_CPPFLAGS, "$CPPFLAGS $DUNE_PKG_CPPFLAGS")
+  AC_LANG_POP([C++])
 ])
 
 AC_DEFUN([DUNE_SUMMARY_ALL],[