diff --git a/Makefile b/Makefile
index 5592488..607d484 100644
--- a/Makefile
+++ b/Makefile
@@ -123,20 +123,25 @@
 Q=
 endif
 
+# WARNING: Do not change cc-opt, cc-opt-alt or cc-warning without checking if
+#          clang bug #49364 is fixed. stderr is redirected to /dev/null on
+#          purpose, to work around a clang 11 bug that crashes if stderr is
+#          redirected to stdin.
+#
 # Function used to detect support of a given option by the compiler.
 # Usage: CFLAGS += $(call cc-opt,option). Eg: $(call cc-opt,-fwrapv)
 # Note: ensure the referencing variable is assigned using ":=" and not "=" to
 #       call it only once.
-cc-opt = $(shell set -e; if $(CC) -Werror $(1) -E -xc - -o /dev/null </dev/null >&0 2>&0; then echo "$(1)"; fi;)
+cc-opt = $(shell set -e; if $(CC) -Werror $(1) -E -xc - -o /dev/null </dev/null >&0 2>/dev/null; then echo "$(1)"; fi;)
 
 # same but emits $2 if $1 is not supported
-cc-opt-alt = $(shell set -e; if $(CC) -Werror $(1) -E -xc - -o /dev/null </dev/null >&0 2>&0; then echo "$(1)"; else echo "$(2)"; fi;)
+cc-opt-alt = $(shell set -e; if $(CC) -Werror $(1) -E -xc - -o /dev/null </dev/null >&0 2>/dev/null; then echo "$(1)"; else echo "$(2)"; fi;)
 
 # Disable a warning when supported by the compiler. Don't put spaces around the
 # warning! And don't use cc-opt which doesn't always report an error until
 # another one is also returned.
 # Usage: CFLAGS += $(call cc-nowarn,warning). Eg: $(call cc-opt,format-truncation)
-cc-nowarn = $(shell set -e; if $(CC) -Werror -W$(1) -E -xc - -o /dev/null </dev/null >&0 2>&0; then echo "-Wno-$(1)"; fi;)
+cc-nowarn = $(shell set -e; if $(CC) -Werror -W$(1) -E -xc - -o /dev/null </dev/null >&0 2>/dev/null; then echo "-Wno-$(1)"; fi;)
 
 #### Installation options.
 DESTDIR =