"distribution-friendly" Makefile

Hello,

As discussed a few days ago on #cmpwn, here a simple patch making
scdoc's Makefile a bit more distribution-friendly.

I don't like the enforced `-static` and would prefer it to be set by the
user if they want it (distributions don't), but that's your call.

Regards,

PS: scdoc is now packaged in Fedora :-)

--
Timothée Floure

Makefile

@@ -1,12 +1,11 @@
 VERSION=1.3.4
-CFLAGS=-g -DVERSION='"$(VERSION)"' -Wall -Wextra -Werror -Wno-unused-parameter
-LDFLAGS=-static
-INCLUDE=-Iinclude
-PREFIX=/usr/local
-DESTDIR=
+CFLAGS+=-g -DVERSION='"$(VERSION)"' -Wall -Wextra -Werror -Wno-unused-parameter
+LDFLAGS+=-static
+INCLUDE+=-Iinclude
+PREFIX?=/usr/local
 _INSTDIR=$(DESTDIR)$(PREFIX)
-BINDIR=$(_INSTDIR)/bin
-MANDIR=$(_INSTDIR)/share/man
+BINDIR?=$(_INSTDIR)/bin
+MANDIR?=$(_INSTDIR)/share/man
 OUTDIR=.build
 HOST_SCDOC=./scdoc
 .DEFAULT_GOAL=all