build.rs: Add linux_time_bits64 to ALLOWED_CFGS

linux_time_bits64 will be used to match __USE_TIME_BITS64 in the uapi
headers.

The environment variable RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64 can be
used by callers to enable the linux_time_bits64 config.

(backport <rust-lang#4148>)
(cherry picked from commit 5453fe6)

Author: snogge

build.rs

@@ -18,6 +18,8 @@ const ALLOWED_CFGS: &'static [&'static str] = &[
     "libc_deny_warnings",
     "libc_thread_local",
     "libc_ctest",
+    // Corresponds to `__USE_TIME_BITS64` in UAPI
+    "linux_time_bits64",
 ];
 
 // Extra values to allow for check-cfg.
@@ -43,6 +45,7 @@ fn main() {
     let rustc_dep_of_std = env::var("CARGO_FEATURE_RUSTC_DEP_OF_STD").is_ok();
     let libc_ci = env::var("LIBC_CI").is_ok();
     let libc_check_cfg = env::var("LIBC_CHECK_CFG").is_ok() || rustc_minor_ver >= 80;
+    let linux_time_bits64 = env::var("RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64").is_ok();
 
     // The ABI of libc used by std is backward compatible with FreeBSD 12.
     // The ABI of libc from crates.io is backward compatible with FreeBSD 11.
@@ -79,6 +82,10 @@ fn main() {
         _ => (),
     }
 
+    if linux_time_bits64 {
+        set_cfg("linux_time_bits64");
+    }
+
     // On CI: deny all warnings
     if libc_ci {
         set_cfg("libc_deny_warnings");

ci/verify-build.sh

@@ -72,6 +72,11 @@ test_target() {
     $cmd --features const-extern-fn
     $cmd --features extra_traits
 
+    if [ "$os" = "linux" ]; then
+        # Test with the equivalent of __USE_TIME_BITS64
+        RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64=1 $cmd
+    fi
+
     # Test again without default features, i.e. without "std"
     $cmd --no-default-features
     $cmd --no-default-features --features extra_traits