Remove uses of LC_GLOBAL_LOCALE

We only use this

1. if we have localeconv_l
2. to get the decimal point / thousands separator for numbers

So we can ignore all this and directly create a purely LC_NUMERIC locale.

This *was* more useful when we were in C++ and the printing functions
all relied on locale, but we only use this in printf and that only
extracts the number stuff.

src/libc.c

@@ -8,11 +8,6 @@
 #include <term.h>
 #include <unistd.h>
 
-// LC_GLOBAL_LOCALE and locale_t are in xlocale.h on macOS
-#ifdef __APPLE__
-#include <xlocale.h>
-#endif
-
 #define UNUSED(x) (void)(x)
 
 size_t C_MB_CUR_MAX() { return MB_CUR_MAX; }
@@ -186,11 +181,3 @@ int C_RLIMIT_NTHR() {
     return -1;
 #endif
 }
-
-locale_t C_LC_GLOBAL_LOCALE() {
-    // LC_GLOBAL_LOCALE is usually -1, but not always (e.g. under NetBSD).
-#ifdef LC_GLOBAL_LOCALE
-    return LC_GLOBAL_LOCALE;
-#endif
-    return (locale_t)-1;
-}

src/libc.rs

@@ -102,7 +102,3 @@ pub fn $cvar() -> $type {
     fn C_RLIMIT_NPTS() -> i32; // ifndef: -1
     fn C_RLIMIT_NTHR() -> i32; // ifndef: -1
 }
-
-extern "C" {
-    pub fn C_LC_GLOBAL_LOCALE() -> libc::locale_t;
-}

src/locale.rs

@@ -65,18 +65,17 @@ unsafe fn read_locale() -> Option<Locale> {
     extern "C" {
         fn localeconv_l(loc: libc::locale_t) -> *const libc::lconv;
     }
-    let LC_GLOBAL_LOCALE: libc::locale_t = unsafe { crate::libc::C_LC_GLOBAL_LOCALE() };
 
     const empty: [libc::c_char; 1] = [0];
-    let cur = libc::duplocale(LC_GLOBAL_LOCALE);
-    if cur.is_null() {
-        return None;
-    }
-    // Note that, counter-intuitively, newlocale() frees 'cur'.
-    let loc = libc::newlocale(libc::LC_NUMERIC_MASK, empty.as_ptr(), cur);
+
+    // We create a new locale (pass 0 locale_t base)
+    // and pass no "locale", so everything else is taken from the environment.
+    // This is fine because we're only using this for numbers.
+    let loc = libc::newlocale(libc::LC_NUMERIC_MASK, empty.as_ptr(), 0 as libc::locale_t);
     if loc.is_null() {
         return None;
     }
+
     let lconv = localeconv_l(loc);
     let result = if lconv.is_null() {
         None