shellinabox: fix build on musl

This commit fixes the build of shellinabox with the musl C library,
which requires two changes:

 - A patch to include <sys/ttydefaults.h>, which is needed to get the
   definitions of TTYDEF_*

 - A hack to workaround what seems to be a problem in musl itself (musl
   does #define utmp utmpx, which causes some symbol conflicts down the
   road). Since anyway the utmpx implementation is just a set of stubs
   in musl, we simply make shellinabox believe that <utmpx.h> is not
   available by passing the appropriate variable. The musl issue has
   been reported at http://www.openwall.com/lists/musl/2016/08/04/8.

Fixes:

  http://autobuild.buildroot.net/results/1847cab964957da3c9bf4911a5ad3602b3c82431/



Signed-off-by: Olivier Singla <olivier.singla@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>