improve error message when using makefile on Mac OSX

[skip ci]

README.md

@@ -58,7 +58,7 @@ There are several `makefile`s provided. Please choose the one that fits best for
 
 | makefile | use-case |
 | -------- | -------- |
-| `makefile` | builds a static library (GNU Make required) |
+| `makefile` | builds a static library (GNU Make required, broken on Mac OSX - use `makefile.unix` instead) |
 | `makefile.shared` | builds a shared (and static) library (GNU Make required) |
 | `makefile.unix` | for unusual UNIX platforms, or if you do not have GNU Make |
 | `makefile.mingw` | for usage with the mingw compiler on MS Windows |

makefile

@@ -17,7 +17,7 @@ PLATFORM := $(shell uname | sed -e 's/_.*//')
 
 ifneq ($(MAKECMDGOALS),clean)
 ifeq ($(PLATFORM), Darwin)
-$(error Can't build static library on Mac, please use makefile.shared)
+$(error Known to not work on Mac, please use makefile.unix for static libraries or makefile.shared for shared libraries)
 endif
 endif