configure: Hardcoded `am__api_version='1.14'` breaks build on systems with autoconf 1.15

[marcusmueller]

… which should be fairly common these days, considering 1.15 is from 2014.

I suspect

am__api_version=`aclocal --version|head -n1|grep -oE '[0-9]+.[0-9]+$'`

would fix the issue, but I'm not sure glog wouldn't want to check the version to be "high" enough.

Reason of problem:

the configure script appends the am__api_version to the aclocal executable name, and of course, on a system with aclocal-1.15, there's no aclocal-1.14.

[ldemailly]

workaround: #117 (comment)