diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 4abe249afe73..19f3bc88f02c 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,9 @@
+2000-06-23  Benjamin Kosnik  <bkoz@purist.soma.redhat.com>
+            Ulrich Drepper  <drepper@cygnus.com>
+	
+	* acinclude.m4 (GLIBCPP_CHECK_MATH_SUPPORT): Add -D_GNU_SOURCE, so
+	that all the freaky stuff like sincos will be around...
+
 2000-06-22  Branko Cibej  <branko.cibej@hermes.si>
 
         * Makefile.am (AM_MAKEFLAGS): Added WERROR to list of flags.
diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index b8d3337054de..4203ea6910c5 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -482,7 +482,7 @@ dnl GLIBCPP_CHECK_MATH_SUPPORT
 AC_DEFUN(GLIBCPP_CHECK_MATH_SUPPORT, [
   ac_test_CXXFLAGS="${CXXFLAGS+set}"
   ac_save_CXXFLAGS="$CXXFLAGS"
-  CXXFLAGS='-fno-builtins'
+  CXXFLAGS='-fno-builtins -D_GNU_SOURCE'
 
   dnl Check libm
   AC_CHECK_LIB(m, sin, libm="-lm")
diff --git a/libstdc++-v3/aclocal.m4 b/libstdc++-v3/aclocal.m4
index 41ca7a46dc7b..a3d5bcb822a1 100644
--- a/libstdc++-v3/aclocal.m4
+++ b/libstdc++-v3/aclocal.m4
@@ -494,7 +494,7 @@ dnl GLIBCPP_CHECK_MATH_SUPPORT
 AC_DEFUN(GLIBCPP_CHECK_MATH_SUPPORT, [
   ac_test_CXXFLAGS="${CXXFLAGS+set}"
   ac_save_CXXFLAGS="$CXXFLAGS"
-  CXXFLAGS='-fno-builtins'
+  CXXFLAGS='-fno-builtins -D_GNU_SOURCE'
 
   dnl Check libm
   AC_CHECK_LIB(m, sin, libm="-lm")
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index 80c621a86d5b..cd0ef68918d1 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -3970,7 +3970,7 @@ EOF
   
   ac_test_CXXFLAGS="${CXXFLAGS+set}"
   ac_save_CXXFLAGS="$CXXFLAGS"
-  CXXFLAGS='-fno-builtins'
+  CXXFLAGS='-fno-builtins -D_GNU_SOURCE'
 
     echo $ac_n "checking for sin in -lm""... $ac_c" 1>&6
 echo "configure:3977: checking for sin in -lm" >&5