Try #165:

bors[bot] · web-flow · commit 2170b63f9eb6 · 2021-10-18T10:03:10.000Z
diff --git a/mbedtls-sys/build/build.rs b/mbedtls-sys/build/build.rs
@@ -99,6 +99,7 @@ impl BuildConfig {
         let mut cflags = vec![];
         if FEATURES.have_platform_component("c_compiler", "freestanding") {
             cflags.push("-fno-builtin".into());
+            cflags.push("-U_FORTIFY_SOURCE".into());
             cflags.push("-D_FORTIFY_SOURCE=0".into());
             cflags.push("-fno-stack-protector".into());
         }

Original file line number	Diff line number	Diff line change
`@@ -99,6 +99,7 @@ impl BuildConfig {`
`99`	`99`	`let mut cflags = vec![];`
`100`	`100`	`if FEATURES.have_platform_component("c_compiler", "freestanding") {`
`101`	`101`	`cflags.push("-fno-builtin".into());`
	`102`	`+ cflags.push("-U_FORTIFY_SOURCE".into());`
`102`	`103`	`cflags.push("-D_FORTIFY_SOURCE=0".into());`
`103`	`104`	`cflags.push("-fno-stack-protector".into());`
`104`	`105`	`}`