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

include/os_compat.h

@@ -111,6 +111,17 @@ void os_localtime(const time_t *t, struct tm *out);
 int os_getpid(void);
 int os_chmod_private(const char *path); /* best-effort 0600; no-op/ok on Win */
 
+/* --- libc gaps on Windows ------------------------------------------------
+ * MinGW-w64 provides neither strcasestr() (a GNU extension) nor
+ * setenv()/unsetenv() (POSIX): they are simply absent from its headers.
+ * src/tui.c uses all three. Implemented in os_compat.c for _WIN32 only,
+ * so the Unix builds keep using the libc versions. */
+#if defined(_WIN32)
+char *strcasestr(const char *haystack, const char *needle);
+int setenv(const char *name, const char *value, int overwrite);
+int unsetenv(const char *name);
+#endif
+
 #ifdef __cplusplus
 }
 #endif