ghostscript: needs host-zlib

ghostscript: needs host-zlib

The ghostscript configure script checks for zlib on the host when
configuring the build of auxiliary programs used in the compilation
process and fails if such library is not found.

Add 'host-zlib' as a dependency and, while at it, also enclose $(HOSTCC)
in double quotes, otherwise configure fails when that variable expands
to a command containing spaces (e.g. when ccache is enabled).

Fixes:

  http://autobuild.buildroot.net/results/61af9caa7fe6fce6b11f78f66b9d3590f37e92e7/
  http://autobuild.buildroot.net/results/904f06e14247c22f740d8fd8902618017e0518f6/

Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>