Literal square brackets must be doubled in configure.ac
Fixes failures from the nightlies:
checking for PYTHON... no
checking for PYTHON... no
File "<string>", line 1
import sys; print(".".join(sys.version.split(" ")0.split(".")[:2]))
^
SyntaxError: invalid syntax
checking for PYTHON... no
checking for PYTHON... no
File "<string>", line 1
import sys; print(".".join(sys.version.split(" ")0.split(".")[:2]))
^