Fix finite-math-only lgamma functions signgam setting (bug 19211)

[Joseph Myers]

On Mon, 14 Mar 2016, Andreas Schwab wrote:

> > Presumably some architecture-specific fix is required for whatever the 
> > issue is with building with that combination of macros
> 
> No, you cannot just #undef _LIBC.

Well, it can be fragile, but it's already done in e.g. 
stdio-common/scanf17.c, another test that tries to avoid internal header 
contents.  I believe we have consensus that it would be desirable for most 
tests not to get _LIBC defined, or include internal headers, at all.  And 
for -std=c99 builds it can be necessary to undefine such macros.  If it 
breaks, we identify the specific cause and add other _LIBC / _ISOMAC 
conditionals, or undefine more macros, as needed.

-- 
Joseph S. Myers
joseph@codesourcery.com