Define __TERMUX__ and __TERMUX_PREFIX__

By defining __TERMUX__ and __TERMUX_PREFIX__ in <sys/cdefs.h> (which is
basically always included) one can more easily target Termux
specifically, both when cross-compiling packages and when code is built
on-device.

ndk-patches/sys-cdefs.h.patch

@@ -1,7 +1,7 @@
-diff -u -r /home/fornwall/lib/android-ndk/sysroot/usr/include/sys/cdefs.h ./usr/include/sys/cdefs.h
---- /home/fornwall/lib/android-ndk/sysroot/usr/include/sys/cdefs.h	2017-07-21 11:04:10.000000000 +0200
-+++ ./usr/include/sys/cdefs.h	2017-08-07 22:50:14.093361547 +0200
-@@ -218,7 +218,11 @@
+diff -u -r /home/builder/lib/android-ndk/sysroot/usr/include/sys/cdefs.h ./usr/include/sys/cdefs.h
+--- /home/builder/lib/android-ndk/sysroot/usr/include/sys/cdefs.h	2019-05-22 12:49:28.000000000 +0000
++++ ./usr/include/sys/cdefs.h	2019-09-19 19:18:03.978640526 +0000
+@@ -176,7 +176,11 @@
   * _FILE_OFFSET_BITS 64 support.
   * See https://android.googlesource.com/platform/bionic/+/master/docs/32-bit-abi.md
   */
@@ -14,3 +14,10 @@ diff -u -r /home/fornwall/lib/android-ndk/sysroot/usr/include/sys/cdefs.h ./usr/
  #  define __USE_FILE_OFFSET64 1
  /*
   * Note that __RENAME_IF_FILE_OFFSET64 is only valid if the off_t and off64_t
+@@ -336,3 +340,6 @@
+ 
+ #include <android/versioning.h>
+ #include <android/api-level.h>
++
++#define __TERMUX__ 1
++#define __TERMUX_PREFIX__ "@TERMUX_PREFIX@"

packages/ndk-sysroot/build.sh

@@ -2,7 +2,7 @@ TERMUX_PKG_HOMEPAGE=https://developer.android.com/tools/sdk/ndk/index.html
 TERMUX_PKG_DESCRIPTION="System header and library files from the Android NDK needed for compiling C programs"
 TERMUX_PKG_LICENSE="NCSA"
 TERMUX_PKG_VERSION=$TERMUX_NDK_VERSION
-TERMUX_PKG_REVISION=1
+TERMUX_PKG_REVISION=2
 TERMUX_PKG_SKIP_SRC_EXTRACT=true
 # This package has taken over <pty.h> from the previous libutil-dev
 # and iconv.h from libandroid-support-dev: