automake: make the build less chatty

Having the "Entering|Leaving directory X" messages it not required nor
useful in vast majority of the cases.

One can always have them printed by `make -w' or by overriding the
AM_MAKEFLAGS variable.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/Makefile.am b/Makefile.am
index 2e46bde..dfb8fcd 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -22,6 +22,7 @@
 
 ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
 
+AM_MAKEFLAGS = -s
 AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--enable-udev \
 	--enable-libkms \