Some cmake version complains about an illegal character

I got the following error:

CMake Error at packages/python.cmake:53 (if): Syntax error in cmake code at

akantu/packages/python.cmake:53

when parsing string

\.a$

Invalid escape sequence .

Which I fixed according to the following forum discussion:

https://stackoverflow.com/questions/50284283/cmake-file-command-gives-invalid-escape-sequence-for-regex-rule