diff --git a/ChangeLog b/ChangeLog
index 6c003f25f..fe18138e1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2011-10-14  Richard Frith-Macdonald <rfm@gnu.org>
+
+	* Source/unix/Makefile.preamble:
+	* Source/win32/Makefile.preamble:
+	* Source/ObjectiveC2/Makefile.preamble:
+	* Source/Additions/Makefile.preamble:
+	* Source/Makefile.preamble:
+	Define _REENTRANT as well as _XOPEN_SOURCE for maximum portability.
+
 2011-10-14  Richard Frith-Macdonald <rfm@gnu.org>
 
 	* Source/NSBundle.m:
diff --git a/Source/Additions/Makefile.preamble b/Source/Additions/Makefile.preamble
index 501095a92..00cf71dc0 100644
--- a/Source/Additions/Makefile.preamble
+++ b/Source/Additions/Makefile.preamble
@@ -40,7 +40,7 @@
 
 # Additional flags to pass to the preprocessor
 ADDITIONAL_CPPFLAGS = $(DEFS) \
--D_XOPEN_SOURCE=600 \
+-D_XOPEN_SOURCE=600 -D_REENTRANT\
 $(WARN_FLAGS) -DNO_GNUSTEP=1
 
 # Additional flags to pass to the Objective-C compiler
diff --git a/Source/Makefile.preamble b/Source/Makefile.preamble
index f71b6d245..f31cdf8e0 100644
--- a/Source/Makefile.preamble
+++ b/Source/Makefile.preamble
@@ -40,7 +40,7 @@
 
 # Additional flags to pass to the preprocessor
 ADDITIONAL_CPPFLAGS = $(DEFS) $(CONFIG_SYSTEM_DEFS) \
--D_XOPEN_SOURCE=600 \
+-D_XOPEN_SOURCE=600 -D_REENTRANT\
 $(WARN_FLAGS) \
 -Wcast-align \
 #-Wstrict-prototypes \
diff --git a/Source/ObjectiveC2/Makefile.preamble b/Source/ObjectiveC2/Makefile.preamble
index d5ee96921..45f8d978c 100644
--- a/Source/ObjectiveC2/Makefile.preamble
+++ b/Source/ObjectiveC2/Makefile.preamble
@@ -40,7 +40,7 @@
 
 # Additional flags to pass to the preprocessor
 ADDITIONAL_CPPFLAGS = $(DEFS) \
--D_XOPEN_SOURCE=600 \
+-D_XOPEN_SOURCE=600 -D_REENTRANT\
 $(WARN_FLAGS)
 
 # Additional flags to pass to the Objective-C compiler
diff --git a/Source/unix/Makefile.preamble b/Source/unix/Makefile.preamble
index 63952778d..12dd80990 100644
--- a/Source/unix/Makefile.preamble
+++ b/Source/unix/Makefile.preamble
@@ -40,7 +40,7 @@
 
 # Additional flags to pass to the preprocessor
 ADDITIONAL_CPPFLAGS = $(DEFS) \
--D_XOPEN_SOURCE=600 \
+-D_XOPEN_SOURCE=600 -D_REENTRANT\
 $(WARN_FLAGS)
 
 # Additional flags to pass to the Objective-C compiler
diff --git a/Source/win32/Makefile.preamble b/Source/win32/Makefile.preamble
index 63952778d..12dd80990 100644
--- a/Source/win32/Makefile.preamble
+++ b/Source/win32/Makefile.preamble
@@ -40,7 +40,7 @@
 
 # Additional flags to pass to the preprocessor
 ADDITIONAL_CPPFLAGS = $(DEFS) \
--D_XOPEN_SOURCE=600 \
+-D_XOPEN_SOURCE=600 -D_REENTRANT\
 $(WARN_FLAGS)
 
 # Additional flags to pass to the Objective-C compiler