Removed uppercase DEBUG, ENABLE_SYMBOLS, DBGLEVEL make variables

[Stephan Bergmann]

On recent master, to simplify the code somewhat, I removed the redundant 
all-uppercase variants of some make variables (e.g., `make DEBUG=t` -> 
`make debug=t`), and only left the all-lowercase variants:

> diff --git a/solenv/gbuild/gbuild.help.txt b/solenv/gbuild/gbuild.help.txt
> index c677541309f5..647bd74afea1 100644
> --- a/solenv/gbuild/gbuild.help.txt
> +++ b/solenv/gbuild/gbuild.help.txt
> @@ -110,11 +110,9 @@ INTERACTIVE VARIABLES:
>         BUILDTOOLTRACE  Run all commands that invoke built tools in strace,
>                         valgrind or a debugger:
>                         BUILDTOOLTRACE='$(DEVENV) /debugexe' PARALLELISM=1 make
> -       DEBUG / debug   If not empty, build as with --enable-debug.
> -       ENABLE_SYMBOLS / enable_symbols
> -                       If not empty, build as with --enable-symbols.
> -       DBGLEVEL / dbglevel
> -                       If not empty, force the debug level to the specified value. The
> +       debug           If not empty, build as with --enable-debug.
> +       enable_symbols  If not empty, build as with --enable-symbols.
> +       dbglevel        If not empty, force the debug level to the specified value. The
>                         debug level is passed to the source code through OSL_DEBUG_LEVEL
>                         macro.
>                         0 = no debug (as with --disable-debug)

(Done as one small step of the overall quest to cut down on our somewhat 
confusing zoo of debugging-related configure options, make variables, 
and C/C++ macros.)