diff --git a/core/Makefile b/core/Makefile
index f153a4f7..b43a9e52 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -57,13 +57,21 @@ endif
 
 
 ifdef USE_SPANDSP
+ifneq ($(spandsp_defs), 1)
+spandsp_defs=1
+export spandsp_defs
 CPPFLAGS += -DUSE_SPANDSP -D__STDC_LIMIT_MACROS
 endif
+endif
 
 ifdef USE_LIBSAMPLERATE
+ifneq ($(libsrc_defs), 1)
+libsrc_defs=1
+export libsrc_defs
 CPPFLAGS += -DUSE_LIBSAMPLERATE
 LDFLAGS +=-lsamplerate
 endif
+endif
 
 ifdef WITH_ZRTP
 LDFLAGS +=-lzrtp -lbn