We currently use static libraries without any source files to represent
header libraries, but Soong actually has cc_library_headers. So to
export those in a separate namespace from static libraries, implement
them in Make as well.
This also adds a nice pretty-warning / pretty-error macro that can be
used to print out standard warning messages pointing to the real source
file having the problem.
Test: Use a header library exported by Soong in a Make module
Change-Id: I3486539e247524cb82a20620745fc7be03014e14
