build: allow empty prefix

there was some concern that this will break windows' way of doing
file lookups relatively from the binary, rather than via hardcoded
locations, but so far each occurence adding e.g. "CA65_INC" to
the pathsearch is already shielded with an #ifndef _WIN32.

addressing #1726

doc/Makefile

@@ -46,7 +46,6 @@ clean:
 	$(RM) -r ../html ../info
 
 install:
-	$(if $(PREFIX),,$(error variable "PREFIX" must be set))
 ifeq ($(wildcard ../html),../html)
 	$(INSTALL) -d $(DESTDIR)$(htmldir)
 	$(INSTALL) -m0644 ../html/*.* $(DESTDIR)$(htmldir)

libsrc/Makefile

@@ -96,7 +96,6 @@ INSTALL = install
 
 define INSTALL_recipe
 
-$(if $(PREFIX),,$(error variable "PREFIX" must be set))
 $(INSTALL) -d $(DESTDIR)$(datadir)/$(dir)
 $(INSTALL) -m0644 ../$(dir)/*.* $(DESTDIR)$(datadir)/$(dir)
 

samples/Makefile

@@ -525,7 +525,6 @@ INSTALL = install
 samplesdir = $(PREFIX)/share/cc65/samples
 
 install:
-	$(if $(PREFIX),,$(error variable "PREFIX" must be set))
 	$(INSTALL) -d $(DESTDIR)$(samplesdir)
 	$(INSTALL) -d $(DESTDIR)$(samplesdir)/geos
 	$(INSTALL) -d $(DESTDIR)$(samplesdir)/tutorial

src/Makefile

@@ -26,7 +26,7 @@ PROGS = ar65     \
 .SUFFIXES:
 
 bindir  := $(PREFIX)/bin
-datadir := $(if $(PREFIX),$(PREFIX)/share/cc65,$(abspath ..))
+datadir := $(PREFIX)/share/cc65
 
 CA65_INC = $(datadir)/asminc
 CC65_INC = $(datadir)/include
@@ -111,7 +111,6 @@ $(RM) /usr/local/bin/$(prog)
 endef # UNAVAIL_recipe
 
 install:
-	$(if $(PREFIX),,$(error variable "PREFIX" must be set))
 	$(INSTALL) -d $(DESTDIR)$(bindir)
 	$(INSTALL) ../bin/* $(DESTDIR)$(bindir)