diff --git a/Makefile b/Makefile
index 6a06ec33..f7ebab39 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 SRCS = mongoose.c test/unit_test.c test/packed_fs.c
 HDRS = $(wildcard src/*.h)
 DEFS ?= -DMG_MAX_HTTP_HEADERS=7 -DMG_ENABLE_LINES -DMG_ENABLE_PACKED_FS=1
-WARN ?= -W -Wall -Werror -Wshadow -Wdouble-promotion -fno-common -Wconversion -Wmissing-prototypes
+WARN ?= -W -Wall -Werror -Wshadow -Wdouble-promotion -fno-common -Wconversion -Wmissing-prototypes -Wundef
 OPTS ?= -O3 -g3
 INCS ?= -Isrc -I.
 CFLAGS ?= $(OPTS) $(WARN) $(INCS) $(DEFS) $(TFLAGS) $(EXTRA)
diff --git a/mongoose.c b/mongoose.c
index 4b18ad6b..59398ad8 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -689,8 +689,9 @@ static size_t p_write(void *fp, const void *buf, size_t len) {
 }
 
 static size_t p_seek(void *fp, size_t offset) {
-#if _FILE_OFFSET_BITS == 64 || _POSIX_C_SOURCE >= 200112L || \
-    _XOPEN_SOURCE >= 600
+#if (defined(_FILE_OFFSET_BITS) && _FILE_OFFSET_BITS == 64) || \
+  (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L) || \
+  (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 600)
   fseeko((FILE *) fp, (off_t) offset, SEEK_SET);
 #else
   fseek((FILE *) fp, (long) offset, SEEK_SET);
@@ -1867,7 +1868,7 @@ void mg_log_set_callback(void (*fn)(const void *, size_t, void *), void *fnd) {
 #include <string.h>
 
 
-#if MG_ENABLE_MD5
+#if defined(MG_ENABLE_MD5) && MG_ENABLE_MD5
 #if !defined(BYTE_ORDER) && defined(__BYTE_ORDER)
 #define BYTE_ORDER __BYTE_ORDER
 #ifndef LITTLE_ENDIAN
@@ -3158,7 +3159,7 @@ static void setsockopts(struct mg_connection *c) {
   setsockopt(FD(c), SOL_TCP, TCP_QUICKACK, (char *) &on, sizeof(on));
 #endif
   setsockopt(FD(c), SOL_SOCKET, SO_KEEPALIVE, (char *) &on, sizeof(on));
-#if ESP32 || ESP8266 || defined(__linux__)
+#if (defined(ESP32) && ESP32) || (defined(ESP8266) && ESP8266) || defined(__linux__)
   int idle = 60;
   setsockopt(FD(c), IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(idle));
 #endif
@@ -3699,7 +3700,7 @@ void mg_timer_poll(unsigned long now_ms) {
 #endif
 #endif
 
-#if MBEDTLS_VERSION_NUMBER >= 0x03000000
+#if defined(MBEDTLS_VERSION_NUMBER) && MBEDTLS_VERSION_NUMBER >= 0x03000000
 #define RNG , rng_get, NULL
 #else
 #define RNG
@@ -3749,7 +3750,7 @@ static void debug_cb(void *c, int lev, const char *s, int n, const char *s2) {
   (void) lev;
 }
 
-#if MBEDTLS_VERSION_NUMBER >= 0x03000000
+#if defined(MBEDTLS_VERSION_NUMBER) && MBEDTLS_VERSION_NUMBER >= 0x03000000
 static int rng_get(void *p_rng, unsigned char *buf, size_t len) {
   (void) p_rng;
   mg_random(buf, len);
diff --git a/src/fs_posix.c b/src/fs_posix.c
index 5f17c551..d9b6d3ca 100644
--- a/src/fs_posix.c
+++ b/src/fs_posix.c
@@ -176,8 +176,9 @@ static size_t p_write(void *fp, const void *buf, size_t len) {
 }
 
 static size_t p_seek(void *fp, size_t offset) {
-#if _FILE_OFFSET_BITS == 64 || _POSIX_C_SOURCE >= 200112L || \
-    _XOPEN_SOURCE >= 600
+#if (defined(_FILE_OFFSET_BITS) && _FILE_OFFSET_BITS == 64) || \
+  (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L) || \
+  (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 600)
   fseeko((FILE *) fp, (off_t) offset, SEEK_SET);
 #else
   fseek((FILE *) fp, (long) offset, SEEK_SET);
diff --git a/src/md5.c b/src/md5.c
index 4b1e3d17..5d42a913 100644
--- a/src/md5.c
+++ b/src/md5.c
@@ -1,7 +1,7 @@
 #include <string.h>
 #include "md5.h"
 
-#if MG_ENABLE_MD5
+#if defined(MG_ENABLE_MD5) && MG_ENABLE_MD5
 #if !defined(BYTE_ORDER) && defined(__BYTE_ORDER)
 #define BYTE_ORDER __BYTE_ORDER
 #ifndef LITTLE_ENDIAN
diff --git a/src/sock.c b/src/sock.c
index 7eb60efe..d268ea2d 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -316,7 +316,7 @@ static void setsockopts(struct mg_connection *c) {
   setsockopt(FD(c), SOL_TCP, TCP_QUICKACK, (char *) &on, sizeof(on));
 #endif
   setsockopt(FD(c), SOL_SOCKET, SO_KEEPALIVE, (char *) &on, sizeof(on));
-#if ESP32 || ESP8266 || defined(__linux__)
+#if (defined(ESP32) && ESP32) || (defined(ESP8266) && ESP8266) || defined(__linux__)
   int idle = 60;
   setsockopt(FD(c), IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(idle));
 #endif
diff --git a/src/tls.c b/src/tls.c
index 9c7d8fac..04c36f6e 100644
--- a/src/tls.c
+++ b/src/tls.c
@@ -16,7 +16,7 @@
 #endif
 #endif
 
-#if MBEDTLS_VERSION_NUMBER >= 0x03000000
+#if defined(MBEDTLS_VERSION_NUMBER) && MBEDTLS_VERSION_NUMBER >= 0x03000000
 #define RNG , rng_get, NULL
 #else
 #define RNG
@@ -66,7 +66,7 @@ static void debug_cb(void *c, int lev, const char *s, int n, const char *s2) {
   (void) lev;
 }
 
-#if MBEDTLS_VERSION_NUMBER >= 0x03000000
+#if defined(MBEDTLS_VERSION_NUMBER) && MBEDTLS_VERSION_NUMBER >= 0x03000000
 static int rng_get(void *p_rng, unsigned char *buf, size_t len) {
   (void) p_rng;
   mg_random(buf, len);