diff --git a/config/CONFIG_COMMON b/config/CONFIG_COMMON
index 157657f35..2994e3222 100644
--- a/config/CONFIG_COMMON
+++ b/config/CONFIG_COMMON
@@ -78,7 +78,8 @@ EPICS_BASE_BIN          = $(EPICS_BASE)/bin/$(T_A)
 GCC_ANSI = $(GCC) -ansi
 GCC_STRICT = $(GCC) -ansi -pedantic
 GCC_TRAD = $(GCC) -traditional
-GCC_DEP_CFLAGS = -fPIC -D_REENTRANT
+GCC_DEP_CFLAGS = -fPIC -D_REENTRANT -D_POSIX_C_SOURCE=199506L -D_POSIX_THREADS
+
 GCC_WARN_YES = -Wall
 GCC_WARN_NO = -w
 GCC_OPT_YES = -O3
@@ -94,7 +95,7 @@ G++_NORMAL = $(G++) -ansi -pedantic
 G++_STRICT = $(G++) -ansi -pedantic -Wtraditional
 G++_TRAD = $(G++) -traditional
 G++_TEMPL_INST_FLAG = -DEXPL_TEMPL
-G++_DEP_CFLAGS = -fPIC -D_REENTRANT
+G++_DEP_CFLAGS = -fPIC -D_REENTRANT -D_POSIX_C_SOURCE=199506L -D_POSIX_THREADS
 G++_WARN_YES = -Wall \
       -Wmissing-prototypes -Woverloaded-virtual \
       -Wwrite-strings -Wconversion -Wstrict-prototypes\