diff --git a/configure.in b/configure.in
index cd94f8ead55..843c3de4e5c 100644
--- a/configure.in
+++ b/configure.in
@@ -1863,6 +1863,11 @@ else
   AC_MSG_RESULT(no)
 fi
 
+dnl On the gcc trunk (as of 2001-02-09) _GNU_SOURCE, and thus __USE_GNU,
+dnl are defined when compiling C++ but not C.  Since the result of this
+dnl test is used only in C++, do it in C++.
+AC_LANG_CPLUSPLUS
+
 AC_MSG_CHECKING(for uname.domainname)
 AC_CACHE_VAL(ac_cv_have_uname_domainname_field,
     [AC_TRY_COMPILE([#include <sys/utsname.h>],
@@ -1893,6 +1898,8 @@ else
     AC_MSG_RESULT(no)
 fi
 
+AC_LANG_C
+
 AC_MSG_CHECKING(for 64-bit OS)
 AC_TRY_RUN([ int main () {
     if (sizeof(long) == 8) { return 0; } return 1; } ],