#pragma once usage

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

Reviewing the dlmopen shared patchset, it propose the use of '#pragma once' on
some tests header instead of include guards. I don't see an impending reason to
not use it, however I would like to check if there any issue I am not seeing.

Besides being non standard features, it also does not really fit our requirement
for exported headers, which usually add other headers and these check the include 
guards from 'parent' to emit an error if the these internal headers are included
directly.

However, I don't see why we can't start to use on glibc code itself (including
tests), at least to simplify some internal guards.  I haven't tested it, specially
on some headers that might be used on preprocessor mode to create constants by 
some scripts.

Is there any other issues that might prevent its internal usage?