Small Makefile improvements

- fix old build commands still using OFLAGS
- make everything work with --warn-undefined-variables, which can be useful
  for debugging typos
diff --git a/programs/Makefile b/programs/Makefile
index 47ffa65..abef94e 100644
--- a/programs/Makefile
+++ b/programs/Makefile
@@ -28,6 +28,9 @@
 ifdef SHARED
 SHARED_SUFFIX=.$(DLEXT)
 endif
+else
+EXEXT=
+SHARED_SUFFIX=
 endif
 
 # Zlib shared library extensions:
@@ -174,11 +177,11 @@
 
 ssl/dtls_client$(EXEXT): ssl/dtls_client.c ../library/libmbedtls.a
 	echo   "  CC    ssl/dtls_client.c"
-	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) $(OFLAGS) ssl/dtls_client.c  $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
+	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) ssl/dtls_client.c  $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
 
 ssl/dtls_server$(EXEXT): ssl/dtls_server.c ../library/libmbedtls.a
 	echo   "  CC    ssl/dtls_server.c"
-	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) $(OFLAGS) ssl/dtls_server.c  $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
+	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) ssl/dtls_server.c  $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
 
 ssl/ssl_client1$(EXEXT): ssl/ssl_client1.c ../library/libmbedtls.a
 	echo   "  CC    ssl/ssl_client1.c"
@@ -226,7 +229,7 @@
 
 test/udp_proxy$(EXEXT): test/udp_proxy.c ../library/libmbedtls.a
 	echo   "  CC    test/udp_proxy.c"
-	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) $(OFLAGS) test/udp_proxy.c    $(LOCAL_LDFLAGS)$(LDFLAGS) -o $@
+	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) test/udp_proxy.c    $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
 
 util/pem2der$(EXEXT): util/pem2der.c ../library/libmbedtls.a
 	echo   "  CC    util/pem2der.c"
diff --git a/tests/Makefile b/tests/Makefile
index 73f9187..a33f8a7 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -37,6 +37,9 @@
 ifdef SHARED
 SHARED_SUFFIX=.$(DLEXT)
 endif
+else
+EXEXT=
+SHARED_SUFFIX=
 endif
 
 # Zlib shared library extensions: