* Gprbuild relies on a configuration file listing all available
  compilers (auto.cgpr is used by default).  You may create such a
  file with the gprconfig tool, but if you don't gprbuild will run
  "gprconfig -o auto.cgpr" by itself (transmitting --db options).
* Gprconfig relies on a configuration directory listing all known
  compilers (/usr/share/gprconfig is used by default). You may provide
  your own modified copy with the --db options.
* In case you need to replace the knowledge base:
gprconfig --db- --db yourdb (generates default.gpr that you may check)
gprbuild yourproject.gpr (using default.gpr)
Exhaustive documentation can be found at
http://docs.adacore.com/gprbuild-docs/html/gprbuild_ug.html.

The upstream database lists various compilers in their default version
(like /usr/bin/gcc).  For each language, unless an
explicit choice is made, gprbuild selects the first detected compiler,
starting with the default version.

Be warned that, for projects mixing languages, the default behaviour
may result in mixing incompatible ABIs, for example when gcc->gcc-x.y
and gnatgcc->gcc-z.t.
This specific problem may be prevented with:
# gprconfig --config=Ada --config=C,,,,gnatgcc
# gprbuild project_mixing_ada_and_c.gpr

 -- Nicolas Boulenguez <nicolas@debian.org>, Mon,  7 Sep 2015 22:06:46 +0200