Use CARGO_CFG_TARGET_ENV to determine if the _target_ env ABI is `g…

…nu` (#31)

The unfortunately named `target_cfg` used in a `cfg()` macro / attribute
is a *host* flag (as all other `cfg()`s) when a build script _using
this library_ is compiled for the host.  Instead, the `ENV` of the
compile _target_ should be read from an environment variable (just like
`$TARGET`) when this code is being run _on the host_ to figure out what
the target environment ABI is.

This causes a cross-compile via `xwin` (reported in #30) to invoke the
`glibc` compile test even though windows doesn't provide `glibc`.  When
cross-compiling to `x86_64-pc-windows-msvc` `target_env` will still be
`"gnu"` while `CARGO_CFG_TARGET_ENV` properly represents `msvc`.

Aside from this I do think that the compile-check is valid for MSVC,
the warning in that error even suggests to include `<string.h>`, but the
test `.c` file already does exactly this... Perhaps the arguments set up
by `xwin` and/or the `sccache` wrapper there are incompatible with how
we invoke the compiler for this test?

src/lib.rs

@@ -56,7 +56,6 @@ where
     Err(())
 }
 
-#[cfg(target_env = "gnu")]
 mod glibc {
     use std::{
         env,
@@ -426,8 +425,10 @@ impl Build {
         }
 
         // https://github.com/jean-airoldie/zeromq-src-rs/issues/28
-        #[cfg(target_env = "gnu")]
-        if !has_strlcpy && glibc::has_strlcpy() {
+        if env::var("CARGO_CFG_TARGET_ENV").unwrap() == "gnu"
+            && !has_strlcpy
+            && glibc::has_strlcpy()
+        {
             has_strlcpy = true;
         }