fix build failure with musl libc

--- configure.ac	2015-07-28 23:01:23.742669306 -0300
+++ configure.ac	2015-07-28 23:01:42.039440565 -0300
@@ -609,7 +609,7 @@
 	[openbsd*], [
 		AC_DEFINE(OPENBSD, , [Building on OpenBSD])
 	],
-	[linux-gnu*], [
+	[linux-gnu*|linux-musl*], [
 		AC_DEFINE(LINUX, , [Building on GNU/Linux])
 		AM_CONDITIONAL(SETUIDBINS, true)
 		BIN_OWNER="root"
