Build: Create liblzma.pc in a src/liblzma/Makefile.am.

Build: Create liblzma.pc in a src/liblzma/Makefile.am.

Previously it was done in configure, but doing that goes
against the Autoconf manual. Autoconf requires that it is
possible to override e.g. prefix after running configure
and that doesn't work correctly if liblzma.pc is created
by configure.

A potential downside of this change is that now e.g.
libdir in liblzma.pc is a standalone string instead of
being defined via ${prefix}, so if one overrides prefix
when running pkg-config the libdir won't get the new value.
I don't know if this matters in practice.

Thanks to Vincent Torri.