Building GDB 15.0.91 on MinGW: CPPFLAGS

[Eli Zaretskii <eliz@gnu.org>]

> From: Joel Brobecker <brobecker@adacore.com>
> Date: Sat,  1 Jun 2024 08:59:07 -0700 (PDT)
> X-Spam-Status: No, score=-4.0 required=5.0 tests=BAYES_00, DKIM_SIGNED,
>  DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE,
>  SPF_PASS, TXREP,
>  T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6
> 
> Hello,
> 
> I have just finished creating the gdb-15.0.91 pre-release.
> It is available for download at the following location:
> 
>     https://sourceware.org/pub/gdb/snapshots/branch/gdb-15.0.91.tar.xz
> 
> A gzip'ed version is also available: gdb-15.0.91.tar.gz.
> 
> Please give it a test if you can and report any problems you might find.

I'm trying to build this pretest, and I need help with setting
non-standard CPPFLAGS.  (I need that to specify the
D__USE_MINGW_ANSI_STDIO=1 preprocessor option, because libctf uses the
%z format which needs that.)

I thought it was enough to specify CPPFLAGS in the top-level configure
command:

  CFLAGS='-O2 -gdwarf-4 -g3' CXXFLAGS='-O2 -gdwarf-4 -g3' CPPFLAGS='-D__USE_MINGW_ANSI_STDIO=1' ./configure ...

However, it sounds like this is not working, because "make V=1"
doesn't show the option on the GCC command line.  Here's an example:

  libtool: compile:  gcc -DHAVE_CONFIG_H -I. -D_GNU_SOURCE -I. -I./../include -I./../bfd -I../bfd -DNOBFD=0 -Id:/usr/include -std=gnu99 -Wall -W -Wall -Wno-narrowing -Wwrite-strings -Wmissing-format-attribute -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition -pedantic -Wno-long-long -O2 -gdwarf-4 -g3 -MT libctf_la-ctf-archive.lo -MD -MP -MF .deps/libctf_la-ctf-archive.Tpo -c ctf-archive.c -o libctf_la-ctf-archive.o

Note that CFLAGS I specified to the top-level configure _are_ used.
Why doesn't the same happen with CPPFLAGS?  Is that some libtool trick
or something?  I do see in top-level Makefile this:

  CPPFLAGS_FOR_BUILD = -D__USE_MINGW_ANSI_STDIO=1

PLEASE HELP, I cannot move on without unlocking this mystery.  (And
yes, there are other issues with this build, so I do have more work on
it.)

TIA