[1442] also temporarily set ldflags and cppflags for configure test

configure.ac

@@ -727,6 +727,10 @@ then
 	# On some systems, there is no gtest-config script, which is supposed
 	# to inform us whether we need -lpthread as well (a gtest compile-time
 	# option). So we still need to test that manually.
+	CPPFLAGS_SAVED="$CPPFLAGS"
+	CPPFLAGS="$CPPFLAGS $GTEST_INCLUDES"
+	LDFLAGS_SAVED="$LDFLAGS"
+	LDFLAGS="$LDFLAGS $GTEST_LDFLAGS"
 	LIBS_SAVED=$LIBS
 	LIBS="$LIBS $GTEST_LDADD"
 	AC_MSG_CHECKING([Checking whether gtest tests need -lpthread])
@@ -757,6 +761,8 @@ then
 				# Apparently we can't compile it at all
 				[ AC_MSG_ERROR(unable to compile with gtest) ])
 	])
+	CPPFLAGS=$CPPFLAGS_SAVED
+	LDFLAGS=$LDFLAGS_SAVED
 	LIBS=$LIBS_SAVED
 else
 	GTEST_INCLUDES=