This will also enable SVID stuff, but that appears to be unavoidable.

See the following URIs:
https://lwn.net/Articles/590381/
https://sourceware.org/glibc/wiki/Release/2.20#Deprecation_of__BSD_SOURCE_and__SVID_SOURCE_feature_macros
http://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html#index-_005fDEFAULT_005fSOURCE