pkg-config uses uppercase for its _CFLAGS and _LIBS variable names.
So we follow suit and convert all _cflags and _libs variables to
uppercase for more consistency.

there are still some files left which could go into the
runtime, but I think we will delete most of them once we
are done with the full modularization.

    different makefiles for the main project and ommysql)
- applied fixes from Michael Biebl:
1.) fix failing compilation of ommysql plugin
    (s/ommysql-config.h/config.h/)
2.) fix mysql configure check (although the default is no, we did check for
    the mysql devel files)
3.) Create a separate Makefile.am for the doc files. This cleans up the
    toplevel Makefile.am considerably and makes it much more readable and
    maintainable.
3b) Assign the html doc files to html_DATA. This means, they are installed
    to $(hmtdir), which by autoconf standards is
    $(prefix)/share/doc/$packagename/.
4.) Reformat the SOURCES line to make it better readable and maintainable.