first cut as separating platform specific code
parity with Handmade Hero Day 011 - The Basics of Platform API Design
split the build non-platfrom and platform steps

in order to keep the non-platform specific code pure, I will build
it without any platform commandline CLFAGS or LFLAGS and then link
to it with the platform build step
move exit-on-failure to the platform layer
rename fill_blit_buf_from_virtual to stretch_buffer
rethinking the platfrom boundary, retreat for now
move the game sound stuff out of the platform layer

clarify that the ring-buffer is platform specific but the
population of the sound is up to the non-platform game code

also, back to two-part build because inclusion of math.h before
defining _GNU and _POSIX stuff creates problems