Rework makefiles to simplify setting and choosing build options.

Includes documentation for common build settings, reasonable
defaults, and ability to set common build options in the
environment.

makefile

@@ -1,13 +1,23 @@
-PLAT?= macosx
+# luasocket makefile
+#
+# see src/makefile for description of how to customize the build
+#
+# Targets:
+#   install       install system independent support
+#   install-unix  also install unix-only support
+#   install-both  install both lua5.1 and lua5.2 socket support
+#   print	  print the build settings
+
+PLAT?= linux
 PLATS= macosx linux win32
 
-#------
-# Hopefully no need to change anything below this line
-#
 all: $(PLAT)
 
 $(PLATS) none install install-unix local clean:
-	@cd src; $(MAKE) $@
+	$(MAKE) -C src $@
+
+print:
+	$(MAKE) -C src $@
 
 test:
 	lua test/hello.lua