Fix definition of FNM_CASEFOLD for Illumos/Solaris

Illumos reference (originally included in Solaris):
https://github.com/illumos/illumos-gate/blob/aaceae985c2e78cadef76bf0b7b50ed887ccb3a6/usr/src/head/fnmatch.h#L41

(backport <rust-lang#4004>)
(cherry picked from commit 98a20b3)

src/unix/mod.rs

@@ -314,9 +314,19 @@ pub const ATF_PUBL: ::c_int = 0x08;
 pub const ATF_USETRAILERS: ::c_int = 0x10;
 
 pub const FNM_PERIOD: c_int = 1 << 2;
-pub const FNM_CASEFOLD: c_int = 1 << 4;
 pub const FNM_NOMATCH: c_int = 1;
 
+cfg_if! {
+    if #[cfg(any(
+        target_os = "illumos",
+        target_os = "solaris",
+    ))] {
+        pub const FNM_CASEFOLD: c_int = 1 << 3;
+    } else {
+        pub const FNM_CASEFOLD: c_int = 1 << 4;
+    }
+}
+
 cfg_if! {
     if #[cfg(any(
         target_os = "macos",