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).

Makefile

@@ -6,6 +6,13 @@ PKG_CONFIG ?= pkg-config
 PREFIX ?= /usr/local
 DESTDIR ?=
 UNAME_S := $(shell uname -s 2>/dev/null)
+UNAME_O := $(shell uname -o 2>/dev/null)
+
+# Android (Termux): pthreads live in libc.
+PTHREAD_LIBS := -lpthread
+ifeq ($(UNAME_O),Android)
+PTHREAD_LIBS :=
+endif
 
 # macOS: Homebrew ncurses/curl are keg-only, so point pkg-config at them.
 # (macOS ships GNU make 3.81, whose $(shell) ignores exported variables.)
@@ -41,7 +48,7 @@ PKG_LIBS := $(CURL_LIBS) $(CURSES_LIBS)
 
 CPPFLAGS += -Iinclude -I$(SD_DIR)/include -I$(CJSON_DIR) -D_XOPEN_SOURCE_EXTENDED $(PKG_CFLAGS)
 LDFLAGS += -L$(SD_DIR)
-LDLIBS += -lsashidown -lm $(PKG_LIBS) -lpthread
+LDLIBS += -lsashidown -lm $(PKG_LIBS) $(PTHREAD_LIBS)
 
 CORE_SRC = \
 	$(CJSON_DIR)/cJSON.c \
@@ -91,7 +98,7 @@ TESTS = \
 	tests/test_undo \
 	tests/test_config
 
-TEST_LIBS = -lm $(CURL_LIBS) -lpthread
+TEST_LIBS = -lm $(CURL_LIBS) $(PTHREAD_LIBS)
 
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