Support overrides for bin, lib, and include

Usually bin and include are the same, but for instance lib may want to be
lib64 on Red Hat distros.  Add BINDIR, LIBDIR, and INCLUDEDIR variables
that can be overridden on the command line.

This commit also tweaks the way the libraries are installed, dropping
execute privileges on libhoedown.a, and making sure libhoedown.so is
installed as a symlink.  (The install command doesn't preserve symlinks.)

Makefile

@@ -1,5 +1,8 @@
 CFLAGS = -g -O3 -ansi -pedantic -Wall -Wextra -Wno-unused-parameter
 PREFIX = /usr/local
+BINDIR = $(PREFIX)/bin
+LIBDIR = $(PREFIX)/lib
+INCLUDEDIR = $(PREFIX)/include
 
 HOEDOWN_CFLAGS = $(CFLAGS) -Isrc
 ifneq ($(OS),Windows_NT)
@@ -64,16 +67,19 @@ clean:
 # Installing
 
 install:
-	install -m755 -d $(DESTDIR)$(PREFIX)/lib
-	install -m755 -d $(DESTDIR)$(PREFIX)/bin
-	install -m755 -d $(DESTDIR)$(PREFIX)/include
+	install -m755 -d $(DESTDIR)$(LIBDIR)
+	install -m755 -d $(DESTDIR)$(BINDIR)
+	install -m755 -d $(DESTDIR)$(INCLUDEDIR)
 
-	install -m644 libhoedown.* $(DESTDIR)$(PREFIX)/lib
-	install -m755 hoedown $(DESTDIR)$(PREFIX)/bin
-	install -m755 smartypants $(DESTDIR)$(PREFIX)/bin
+	install -m644 libhoedown.a $(DESTDIR)$(LIBDIR)
+	install -m755 libhoedown.so.3 $(DESTDIR)$(LIBDIR)
+	ln -f -s libhoedown.so.3 $(DESTDIR)$(LIBDIR)/libhoedown.so
 
-	install -m755 -d $(DESTDIR)$(PREFIX)/include/hoedown
-	install -m644 src/*.h $(DESTDIR)$(PREFIX)/include/hoedown
+	install -m755 hoedown $(DESTDIR)$(BINDIR)
+	install -m755 smartypants $(DESTDIR)$(BINDIR)
+
+	install -m755 -d $(DESTDIR)$(INCLUDEDIR)/hoedown
+	install -m644 src/*.h $(DESTDIR)$(INCLUDEDIR)/hoedown
 
 # Generic object compilations