Create and use first-versions.h with macros for function symbol versions

[Florian Weimer]

On 04/21/2017 11:13 AM, Joseph Myers wrote:
> This patch arranges for the glibc build to generate a header
> first-versions.h that defines macros for the earliest symbol version
> in which each public symbol (GLIBC_[0-9]* symbol version, name only
> uses C identifier characters) is available.

Is this the version used in the fourth argument to the compat_symbol 
macro?  Maybe we can use this mechanism to remove that argument, too, to 
avoid potential mistakes there?

Thanks,
Florian