Upgrade buildsystem to look for python, python3 or python2

(cherry picked from commit 181050b)

Make.inc

@@ -1010,13 +1010,17 @@ else
   UTF8PROC_INC := $(build_includedir)
 endif
 
+# We need python for things like BB triplet recognition.  We don't really care
+# about version, generally, so just find something that works:
+PYTHON := $(shell which python 2>/dev/null || which python3 2>/dev/null || which python2 2>/dev/null || echo not found)
+
 # BinaryBuilder options.  We default to "on" for all the projects listed in BB_PROJECTS,
 # but only if contrib/normalize_triplet.py works for our requested triplet.
-ifeq ($(shell python $(JULIAHOME)/contrib/normalize_triplet.py $(or $(XC_HOST),$(XC_HOST),$(BUILD_MACHINE)) >/dev/null 2>/dev/null; echo $$?),0)
+ifeq ($(shell $(PYTHON) $(JULIAHOME)/contrib/normalize_triplet.py $(or $(XC_HOST),$(XC_HOST),$(BUILD_MACHINE)) >/dev/null 2>/dev/null; echo $$?),0)
 USE_BINARYBUILDER ?= 1
 else
-ifneq ($(shell python $(JULIAHOME)/contrib/normalize_triplet.py x86_64-linux-gnu),x86_64-linux-gnu)
-$(warning python normalize_triplet.py appears to be non-functional, so BinaryBuilder disabled)
+ifneq ($(shell $(PYTHON) $(JULIAHOME)/contrib/normalize_triplet.py x86_64-linux-gnu),x86_64-linux-gnu)
+$(warning normalize_triplet.py appears to be non-functional (used python interpreter "$(PYTHON)"), so BinaryBuilder disabled)
 endif
 USE_BINARYBUILDER ?= 0
 endif

deps/tools/bb-install.mk

@@ -5,8 +5,7 @@
 #    4 cxx11)                  # signifies a cxx11 ABI dependency
 
 # Auto-detect triplet once, create different versions that we use as defaults below for each BB install target
-# This is much more efficient than launching `gcc` and `python` once for each BB install target.
-BB_TRIPLET_GCCABI_CXXABI := $(shell python $(JULIAHOME)/contrib/normalize_triplet.py $(or $(XC_HOST),$(XC_HOST),$(BUILD_MACHINE)) "$(shell $(FC) --version | head -1)" "$(shell echo '\#include <string>' | $(CXX) $(CXXFLAGS) -x c++ -dM -E - | grep _GLIBCXX_USE_CXX11_ABI | awk '{ print $$3 }' )")
+BB_TRIPLET_GCCABI_CXXABI := $(shell $(PYTHON) $(JULIAHOME)/contrib/normalize_triplet.py $(or $(XC_HOST),$(XC_HOST),$(BUILD_MACHINE)) "$(shell $(FC) --version | head -1)" "$(shell echo '\#include <string>' | $(CXX) $(CXXFLAGS) -x c++ -dM -E - | grep _GLIBCXX_USE_CXX11_ABI | awk '{ print $$3 }' )")
 BB_TRIPLET_GCCABI := $(subst $(SPACE),-,$(filter-out cxx%,$(subst -,$(SPACE),$(BB_TRIPLET_GCCABI_CXXABI))))
 BB_TRIPLET_CXXABI := $(subst $(SPACE),-,$(filter-out gcc%,$(subst -,$(SPACE),$(BB_TRIPLET_GCCABI_CXXABI))))
 BB_TRIPLET := $(subst $(SPACE),-,$(filter-out cxx%,$(filter-out gcc%,$(subst -,$(SPACE),$(BB_TRIPLET_GCCABI_CXXABI)))))