improve portability of the Makefile

I like to build on Alpine Linux when possible, but
coreutils on Alpine does not include the
[`arch`](http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob_plain;f=src/coreutils-arch.c;hb=HEAD)
wrapper to print the kernel architecture.

`uname` and `arch` are both provided by coreutils rpm
on RedHat derivatives, so this change does not impact
the build on those distros.

Makefile

@@ -28,7 +28,7 @@ ifeq ($(VERSION),$(SVERSION))
 else
 	RELEASE = untagged
 endif
-ARCH = $(shell arch)
+ARCH = $(uname -i)
 SPEC = rpmbuild/SPECS/$(PROGRAM).spec
 SOURCE = rpmbuild/SOURCES/$(PROGRAM)-$(VERSION).tar.bz2
 SRPM = rpmbuild/SRPMS/$(PROGRAM)-$(VERSION)-$(RELEASE).src.rpm