[patch] Don't install libquadmath.info if libquadmath is not actually built

[FX]

The attached patch prevents libquadmath from installing its info file if the 
library is not actually built.
Tested on x86_64-linux, both on a built with libquadmath (normal) and without 
(tweaking configure so it thinks _float128 is not supported).

OK to commit?

quadmath.ChangeLog
Description: Binary data

quadmath.diff
Description: Binary data