can now build Android targeted binary

Various things that don't work on Android are just ifdefed out.

* the webapp (needs template haskell for arm)
* --include and --exclude globbing (needs libpcre, which is not ported;
  probably I'll make it use the pure haskell glob library instead)
* annex.diskreserve checking (missing sys/statvfs.h)
* timestamp preservation support (yawn)
* S3
* WebDAV
* XMPP

The resulting 17mb binary has been tested on Android, and it is able to,
at least, print its usage message.

Utility/libdiskfree.c

@@ -22,6 +22,10 @@
 # define STATCALL statfs /* statfs64 not yet tested on a real FreeBSD machine */
 # define STATSTRUCT statfs
 #else
+#if defined WITH_ANDROID
+# warning free space checking code not available for Android
+# define UNKNOWN
+#else
 #if defined (__linux__) || defined (__FreeBSD_kernel__)
 /* Linux or Debian kFreeBSD */
 /* This is a POSIX standard, so might also work elsewhere too. */
@@ -34,6 +38,7 @@
 #endif
 #endif
 #endif
+#endif
 
 #include <errno.h>
 #include <stdio.h>