Version 2.1.5 introduced simple test phase witch depends on 'compress' command. If there is no compress command on system then test phase fails.
Created attachment 208992 [details] Failing build.log
Created attachment 208994 [details, diff] Fix to ebuild Added 'test' USE flag and conditional dependency on app-arch/ncompress
Fixed in portage, thanks for the patch! + 02 Nov 2009; Víctor Ostorga <vostorga@gentoo.org> -pigz-2.1.4.ebuild, + -files/pigz-2.1.4-respect-flags.patch, pigz-2.1.5.ebuild: + Adding DEPEND on app-arch/ncompress when FEATURES=test , patch thanks to + Raimonds Cicans <ray@apollo.lv> bug 291504