Rename --with-stdcpp → --with-libstdc++

Also improve wording a bit and fix make check in --with-libstdc++=yes.
diff --git a/src/Makefile.am b/src/Makefile.am
index 2572856..7b7ccde 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -30,7 +30,7 @@
 HBHEADERS = $(HB_BASE_headers)
 HBNODISTHEADERS = $(HB_NODIST_headers)
 
-if !WITHSTDCPP
+if !WITH_LIBSTDCXX
 # Make sure we don't link to libstdc++
 HBCFLAGS += -fno-rtti -fno-exceptions
 
@@ -128,7 +128,7 @@
 harfbuzz_subset_def_dependency = harfbuzz-subset.def
 choosed_linker = $(CXXLINK)
 else
-if WITHSTDCPP
+if WITH_LIBSTDCXX
 choosed_linker = $(CXXLINK)
 else
 if HAVE_GCC
@@ -340,11 +340,16 @@
 	check-externs.sh \
 	check-header-guards.sh \
 	check-includes.sh \
-	check-libstdc++.sh \
 	check-static-inits.sh \
 	check-symbols.sh \
 	$(NULL)
 
+if !WITH_LIBSTDCXX
+dist_check_SCRIPTS += \
+	check-libstdc++.sh \
+	$(NULL)
+endif
+
 check_PROGRAMS = \
 	test-ot-tag \
 	$(NULL)