diff --git a/Makefile.am b/Makefile.am
index 65de4f922..f25c0f068 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -24,7 +24,7 @@ pixmap_DATA = htop.png
 appicondir = $(datadir)/icons/hicolor/scalable/apps
 appicon_DATA = htop.svg
 
-AM_CFLAGS += -D_XOPEN_SOURCE_EXTENDED -DSYSCONFDIR="\"$(sysconfdir)\"" -I"$(top_srcdir)/$(my_htop_platform)"
+AM_CFLAGS += -DSYSCONFDIR="\"$(sysconfdir)\"" -I"$(top_srcdir)/$(my_htop_platform)"
 AM_LDFLAGS =
 
 myhtopsources = \
diff --git a/configure.ac b/configure.ac
index 857d865bf..5c9ccd2d4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -68,6 +68,22 @@ esac
 # Enable extensions, required by hwloc scripts
 AC_USE_SYSTEM_EXTENSIONS
 
+# The header of ncurses exposes wide-character interfaces only if
+# _XOPEN_SOURCE_EXTENDED is defined or _XOPEN_SOURCE defined to be
+# >= 500. In DragonFly BSD, FreeBSD and OpenBSD, defining
+# _XOPEN_SOURCE to any value *hides* interfaces that would be useful
+# for htop.
+dnl
+dnl Note: The "#undef" in AH_VERBATIM will be replaced with "#define"
+dnl when config.h is generated.
+AH_VERBATIM([_XOPEN_SOURCE_EXTENDED],
+[/* Enables XPG4v2 (SUSv1) interfaces if they are not enabled already with _XOPEN_SOURCE */
+#ifndef _XOPEN_SOURCE_EXTENDED
+#undef _XOPEN_SOURCE_EXTENDED
+#endif
+])
+AC_DEFINE([_XOPEN_SOURCE_EXTENDED], 1)
+
 # Activate some more of the missing global defines
 AC_SYS_LARGEFILE