build: Use conditionals for OS-specific source files

OS-specific source files were defined in configure.ac through
AC_SUBST, and then they were forced to be dist'ed using
an EXTRA_ Makefile.am variable.
This is quite unusual, now that we have automake conditionals
for each supported OS, we can make use of them.
build: Move headers to libdiscid_la_SOURCES

Header files are sources as well, so we can list them in a _SOURCES
primary rather than in EXTRA_DIST.
build: Use per-target CPPFLAGS

We can set CPPFLAGS in libdiscid_la_CPPFLAGS rather than AM_CPPFLAGS
to be consistent with all the other libdiscid_la_XXXX variables.