Merge #165

165: Unset _FORTIFY_SOURCE before setting it to fix error about redefinition r=jethrogb a=sburton84

When building for the `x86_64-fortanix-unknown-sgx` target in release mode I receive the following errors:

```
  <command-line>: error: "_FORTIFY_SOURCE" redefined [-Werror]
  <built-in>: note: this is the location of the previous definition
  <command-line>: error: "_FORTIFY_SOURCE" redefined [-Werror]
  <built-in>: note: this is the location of the previous definition
  cc1: all warnings being treated as errors
```

I think these would only be warnings but `MBEDTLS_FATAL_WARNINGS` defaults to `ON` which makes them into errors.

This patch fixes this by explicitly unsetting `_FORTIFY_SOURCE` before setting it to a new value.

Co-authored-by: Sean Burton <sean.burton@cambridgequantum.com>

Author: bors[bot]

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());
         }