gprbuild fun

[Simon Wright <simon@pushface.org>]

I have a mixed-language GPR like

   project build is
      for languages use ("ada", "c");
      for source_files use ("a.adb", "c.c");
      package ide is
         for compiler_command ("ada") use "gnatmake";
      end ide;
   end build;

(I have no idea why I introduced the compiler_command setting on
2014-05-01, and I have no idea why the issue has only just surfaced)

and, if I have the native gcc and arm-eabi-gcc present in the PATH,
gprbuild chooses to use the native gcc for the Ada part and arm-eabi-gcc
for the C part!

Fix: delete the compiler_command line.

_That_ was fun.