The default gprbuild configuration supports version 4.9 of all
compilers provided by the GCC suite. This ensures that projects mixing
objects compiled from many languages will share the same ABI.

* 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.

If you need to use a different version of a compiler, say gcc-4.6:
cp -r /usr/share/gprconfig yourdb
in yourdb/compilers.xml, in the <name>GCC</name> stanza,
  replace "gcc" with "gcc-4.6" in line <executable ... gcc</executable>
  and in line <external>${PREFIX}gcc -dumpmachine</external>
  replace the <version>...</version> bloc with <version>4.6</version>
in yourdb/c.xml, in the <configuration> stanza containing
  both a line <compiler name="GCC" language="C" />
  and a line  for Driver              ("C") use "${PATH(c)}${PREFIX(c)}gcc";
  replace "gcc" with "gcc-4.6" in the Driver line above.
gprconfig --db- --db yourdb (generates default.gpr that you may check)
gprbuild yourproject.gpr (using default.gpr)

 -- Nicolas Boulenguez <nicolas@debian.org>, Sat,  8 Nov 2014 15:55:39 +0100