Termux: build with make, install to $PREFIX, no nl_langinfo on Android

- bootstrap_termux.sh installs pkg-config/binutils, builds with make,
  runs the tests and installs into Termux's $PREFIX/bin.
- Makefile drops -lpthread on Android (pthreads are in bionic libc).
- TUI assumes UTF-8 on Android: nl_langinfo needs API 26, Termux
  targets 24. All sources compile for aarch64-linux-android24 (NDK 27).

src/tui.c

@@ -369,7 +369,9 @@ static void theme_init(void) {
   const char *nc = getenv("NO_COLOR");
   g_nocolor = nc && nc[0];
   const char *asc = getenv("CST_ASCII");
-#ifndef _WIN32
+#if defined(__ANDROID__)
+  g_utf8 = 1; /* bionic is UTF-8 only; nl_langinfo needs API 26 */
+#elif !defined(_WIN32)
   const char *cs = nl_langinfo(CODESET);
   g_utf8 = cs && (strcasecmp(cs, "UTF-8") == 0 || strcasecmp(cs, "utf8") == 0);
 #endif