build: rename configure.in -> .ac

"configure.in" is a very, very old, obsolete name. Newer automake
will reject it, so rename it now.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>

diff --git a/configure.in b/configure.ac
similarity index 100%
rename from configure.in
rename to configure.ac

diff --git a/doc/configure.in b/doc/configure.ac
similarity index 100%
rename from doc/configure.in
rename to doc/configure.ac