Remove abspath from native Makefile

It does not seem extremely portable, since it has some
issues at least on windows gmake. It does not recognize
windows absolute path with drive letters (e.g. D:\foo).

https://stackoverflow.com/questions/21288101

Makefile

@@ -74,7 +74,7 @@ else
 endif
 
 ifndef SASS_LIBSASS_PATH
-	SASS_LIBSASS_PATH = $(abspath $(CURDIR))
+	SASS_LIBSASS_PATH = $(CURDIR)
 endif
 ifdef SASS_LIBSASS_PATH
 	CFLAGS   += -I $(SASS_LIBSASS_PATH)/include