Improve PREFIX handling

Honor PREFIX both at configure time and during `make install'. Should
make the recent auto{conf,make} replacement more compliant with GNU
standards[1].

While here, shuffle the default choice of PREFIX to configure, like all
other defaults.

Issue raised by Aggelos Avgerinos in #292.

[1] https://www.gnu.org/prep/standards/html_node/Directory-Variables.html#Directory-Variables

Makefile

@@ -67,8 +67,6 @@ DISTFILES+=	tests/opt-x.t
 DISTFILES+=	tests/pick-test.c
 DISTFILES+=	tests/pick-test.sh
 
-PREFIX=	/usr/local
-
 all: ${PROG}
 
 ${PROG}: ${OBJS}

configure

@@ -140,6 +140,7 @@ INSTALL=$(makevar INSTALL)
 : "${DEBUG:=}"
 : "${INSTALL:=install}"
 : "${LDFLAGS:=}"
+: "${PREFIX:=/usr/local}"
 
 exec 3>&1 4>&2
 exec 1>config.log 2>&1
@@ -224,4 +225,6 @@ DEBUG=		$(echo $DEBUG)
 INSTALL=	$(echo $INSTALL)
 LDFLAGS=	$(echo $LDFLAGS)
 MALLOC_OPTIONS=	$(echo $MALLOC_OPTIONS)
+
+PREFIX?=	$(echo $PREFIX)
 EOF