Caching pkgdatadir in configure

[Alberto Luaces]

Hello,

I wrote a program that uses data files at runtime. I store them in
$pkgdatadir, and it works fine.

Sometimes I want to execute the program without installing it. To do
this, I change the value of the $pkgdatadir variable at make time:

make pkgdatadir=...

I wonder if I could modify and cache $pkgdatadir variable when running
configure, to avoid having to specify the value of pkgdatadir every time
I am recompiling.

I tried to use AC_SUBST(pkgdatadir), but this makes that the $pkgdatadir
value is void if I don't define it when configuring because it overrides
the Automake definition $datadir/$packagename.

Is it possible to do what I want?

1. If pkgdatadir is not specified at configure time, follow the FHS or
GNU guidelines.
2. If pkgdatadir is specified, store its value in the configure cache.

Thank you,

Alberto