Makefile: don't hardcode the path to 'install'

In Arch Linux we currently use /bin/install rather than
/usr/bin/install. This change work with both locations and
avoids us having to change the Makefile.

Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

Makefile

diff --git a/Makefile b/Makefile
index 9d538a9..ea44bb6 100644
--- a/Makefile
+++ b/Makefile
@@ -76,7 +76,7 @@
 mandir =	${prefix}/usr/share/man
 udevdir =	${prefix}/lib/udev
 
-INSTALL = /usr/bin/install -c
+INSTALL = install -c
 INSTALL_PROGRAM = ${INSTALL}
 INSTALL_DATA  = ${INSTALL} -m 644
 INSTALL_SCRIPT = ${INSTALL_PROGRAM}