Provide pkg-config (.pc) scripts

Provide pkg-config (.pc) scripts

This allows a project to use PKG_CHECK_MODULES() in its configure.ac
file to easily check for the presence of libjpeg-turbo and modify the
compiler/linker flags accordingly.  Note that if a project relies solely
on pkg-config to check for libjpeg-turbo, then it will not be possible
to build that project using libjpeg or an earlier version of
libjpeg-turbo.

Closes #53

Based on:
https://github.com/cberner/libjpeg-turbo/commit/496713871939b550d00005b4042420da41641a0a