- Let the build environment decide about the toolchain to be used, in case

there are special tweaks and paths to be considered. Just provide some
  defaults, in case the values are undefined.
- py-version.sh does not need any bash specific features.
- Use libdl only on Linux for now. FreeBSD provides dl*() calls from libc.

py/mkenv.mk

@@ -42,14 +42,14 @@ ECHO = @echo
 CP = cp
 MKDIR = mkdir
 SED = sed
-PYTHON = python
+PYTHON ?= python
 
-AS = $(CROSS_COMPILE)as
-CC = $(CROSS_COMPILE)gcc
-LD = $(CROSS_COMPILE)ld
-OBJCOPY = $(CROSS_COMPILE)objcopy
-SIZE = $(CROSS_COMPILE)size
-STRIP = $(CROSS_COMPILE)strip
+AS ?= $(CROSS_COMPILE)as
+CC ?= $(CROSS_COMPILE)gcc
+LD ?= $(CROSS_COMPILE)ld
+OBJCOPY ?= $(CROSS_COMPILE)objcopy
+SIZE ?= $(CROSS_COMPILE)size
+STRIP ?= $(CROSS_COMPILE)strip
 
 all:
 .PHONY: all