Windows: make the CMake build work with MSYS2's native toolchain

src/os_compat.c

@@ -1168,3 +1168,52 @@ void os_proc_kill(os_proc_t *p) {
   }
 }
 #endif
+
+/* --- libc gaps on Windows -----------------------------------------------
+ * strcasestr() is a GNU extension and setenv()/unsetenv() are POSIX; glibc
+ * has all three, MinGW-w64 has none of them. src/tui.c needs them (command
+ * filtering and ESCDELAY), so the Windows build supplies its own.
+ *
+ * The case folding here is ASCII-only, unlike glibc's locale-aware version.
+ * That is deliberate: these are used to match command and setting names,
+ * which are ASCII.
+ */
+#if defined(_WIN32)
+
+static int ascii_lower(int c) {
+  return (c >= 'A' && c <= 'Z') ? c + ('a' - 'A') : c;
+}
+
+char *strcasestr(const char *haystack, const char *needle) {
+  if (!haystack || !needle)
+    return NULL;
+  if (!*needle)
+    return (char *)haystack;
+  for (; *haystack; haystack++) {
+    const char *h = haystack;
+    const char *n = needle;
+    while (*h && *n && ascii_lower((unsigned char)*h) == ascii_lower((unsigned char)*n)) {
+      h++;
+      n++;
+    }
+    if (!*n)
+      return (char *)haystack;
+  }
+  return NULL;
+}
+
+int setenv(const char *name, const char *value, int overwrite) {
+  if (!name || !*name || strchr(name, '='))
+    return -1;
+  if (!overwrite && getenv(name))
+    return 0;
+  return _putenv_s(name, value ? value : "");
+}
+
+int unsetenv(const char *name) {
+  if (!name || !*name || strchr(name, '='))
+    return -1;
+  return _putenv_s(name, "");
+}
+
+#endif /* _WIN32 */