Port configure script to C99

[NIIBE Yutaka]

Florian Weimer wrote:
> We noticed that libgcrypt fails to build because it uses implicit
> function declarations in the configure script.  The patch below should
> fix that.

Thank you.

Applied and pushed the change (so that it has consistent tab&space) to
master.  Will apply to 1.10 branch, too.

For libgcrypt, I found m4/ax_cc_for_build.m4 and m4/noexecstack.m4 have
similar problems.  I'll fix.

For the source distribution (*.tar.bz2) which has generated configure
script, I think that we need to fix more.  IIUC, Autoconf and Libtool
need to be updated.
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