Rename configure.in to configure.ac

[glensc]

modernize autotools

aclocal: warning: autoconf input should be named 'configure.ac', not 'configure.in'

From Autoconf Manual:

Previous versions of Autoconf promoted the name configure.in, which is somewhat ambiguous (the tool needed to process this file is not described by its extension), and introduces a slight confusion with config.h.in and so on (for which ‘.in’ means “to be processed by configure”). Using configure.ac is now preferred.

There's also two more configure.in in the project (excluding third party libraries in ext/), i would convert those too if this PR isn't immediately rejected:

• TSRM/configure.in
• Zend/configure.in

Also I think it would be reasonable to drop autogenerated content from git, who wishes to build from source (not release tarball) should invoke ./buildconf.

[ghost]

This looks like a duplicate to my #1635 PR

[smalyshev]

looks like duplicate of #1635 - please reopen if I'm wrong.

[glensc]

@smalyshev yes, partially duplicate, i.e my question about commiting autogenerated files is not answered in neither PR's