Using config_xxx.h instead of -DHAVE_XXX

[Bjoern Michaelsen]

On Fri, Dec 07, 2012 at 04:23:36PM -0600, Norbert Thiebaud wrote:
> oh well...I still don't like to pollution of the build root with an
> ever growing series of config_foo.h file... that will need to be added
> to .gitignore... if they were in a config directory that would be much
> neater and much easier to do a safe distro-clean

Yep, a config dir would be much better, for multiple reasons:
- easier for .gitignore
- easier for packagers (apt complains about changes to the source tree)
- easier for a read-only source tree (that dir can e.g. be bind-mounted, a tmpfs etc)
- easier to set as -I path to find the header
- SRCDIR should never be in the include path -- it would lead to all kinds
  trouble wrt which header is chosen (the one from solver?, the one from
  SRCDIR? both will likely have the same relative path), leading to "interesting"
  dep-files, and possibly mysterious rebuild issues ....

Best,

Bjoern