Remove requirement of config.h/qs_config.h in headers
Makefile.am: Install header files for library

The installs all library header files to ${prefix}/include/qsopt_ex.
The QSopt_ex.h header can be used to include all headers, or they
can be included separately. Note that the header files define a lot
of macros that are not namespace-prefixed and these can easily
interfere with the program they are included in.
Makefile.am: Do not install qs_config.h header

Creates a list of headers that are private (not installed) and adds
qs_config.h to this list. Private headers cannot be included by the
public headers, they can only be included by source files.