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:
4967138719
