configure.ac, src/Makefile.am: Follow GNU coding standards regarding directory variables

According to Sam, if we just remove the definitions, autotools will
provide the right values.

Note that this changes the default from /usr to /usr/local, which is the
right value according to the GNU conding standards (and also what the
BSDs prefer, according to one conversation I had with Ingo Schwarze from
OpenBSD).

Closes: <#1229>
Link: <https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Default-Prefix.html>
Reported-by: Chris Hofstaedtler <zeha@debian.org>
Cc: Sam James <sam@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

Author: alejandro-colomar

configure.ac

@@ -21,10 +21,6 @@ AC_SUBST([LIBSUBID_ABI_MINOR], [libsubid_abi_minor])
 AC_SUBST([LIBSUBID_ABI_MICRO], [libsubid_abi_micro])
 AC_SUBST([LIBSUBID_ABI], [libsubid_abi])
 
-dnl Some hacks...
-test "x$prefix" = "xNONE" && prefix="/usr"
-test "X$prefix" = "X/usr" && exec_prefix=""
-
 AC_USE_SYSTEM_EXTENSIONS
 
 AC_ENABLE_STATIC

src/Makefile.am

@@ -2,8 +2,8 @@
 EXTRA_DIST = \
 	.indent.pro
 
-ubindir = ${prefix}/bin
-usbindir = ${prefix}/sbin
+ubindir = ${exec_prefix}/bin
+usbindir = ${exec_prefix}/sbin
 suidperms = 4755
 sgidperms = 2755
 
@@ -24,7 +24,7 @@ AM_CFLAGS = $(LIBBSD_CFLAGS)
 #
 # also /lib/libshadow.so.x.xx (if any) could be moved to /usr/lib
 # and installation would be much simpler (just two directories,
-# $prefix/bin and $prefix/sbin, no install-data hacks...)
+# $exec_prefix/bin and $exec_prefix/sbin, no install-data hacks...)
 
 bin_PROGRAMS   = login
 sbin_PROGRAMS  = nologin