Avoid breakage with gcc 5

* src/Makefile.am: Add -P to the C preprocessor when building
mkerrcodes.h, to avoid a noisy intermediate pipeline.

With gcc 5 without this patch, we see many errors like the following:

gcc -I. -I. -o mkerrcodes ./mkerrcodes.c
In file included from ./mkerrcodes.c:26:0:
./mkerrcodes.h:9:5: error: expected expression before ‘,’ token

{ , "GPG_ERR_E2BIG" },
  ^

./mkerrcodes.h:10:5: error: expected expression before ‘,’ token

{ , "GPG_ERR_EACCES" },
  ^

This patch cleans up the generated mkerrcodes.h by making the
intermediate stage clean for all the versions of gcc i tested (4.x and
5).

• Debian-Bug-Id: #777374
• Signed-Off-By: Daniel Kahn Gillmor <dkg@fifthhorseman.net>