[macos] missing autoconf for libnfs build

[xhaggi]

Got the following error, because autoconf does not exists on MacOS. To build Kodi's depends we build a temporary copy of autoconf. See https://github.com/xbmc/xbmc/blob/master/tools/depends/bootstrap I'm not very familiar with cmake and how we could fix it.

[ 60%] Performing build step for 'nfs'
[ 12%] Performing update step for 'nfs'
./bootstrap: line 2: autoreconf: command not found
make[5]: *** [nfs-prefix/src/nfs-stamp/nfs-update] Error 127
make[4]: *** [CMakeFiles/nfs.dir/all] Error 2
make[3]: *** [all] Error 2
make[2]: *** [build/nfs/src/nfs-stamp/nfs-build] Error 2
make[1]: *** [CMakeFiles/nfs.dir/all] Error 2
make: *** [all] Error 2

[notspiff]

use https://cmake.org/cmake/help/v3.0/command/find_program.html to locate autoconf binary, then call using the returned value. i believe the depends path should be in the prefix paths, so we should be able to locate it that way.