This is a problem with Python, obviously.
I don't know how Arch Linux works, but on RedHat derivatives, there's an "alternatives" command which allows to pick up the version of various programs (such as gcc and python), and you type:
to choose which version will be linked to 'python', 'gcc', 'g++', etc... What 'alternatives' does is to change the links in /etc/alternatives/ to point to the right binaries, those link being themselves pointed to by '/usr/bin/python', '/usr/bin/gcc', etc... I.e, you get (with '->' denoting a soft link):
/usr/bin/python -> /etc/alternatives/python
/etc/alternatives/python -> /usr/bin/python2
This way, when you invoke '/usr/bin/python', it's /usr/bin/python2 which gets executed, and only the links in /etc/alternatives need to be changed to choose which python version to use. Things with gcc are more complex, because 'alternatives' also changes the links to 'cc', 'g++', 'c++', 'cpp', etc...
If you don't have an equivalent of the 'alternatives' utility in Arch Linux, then you still can use your patch, but you missed the indra/cmake/Python.cmake file. Change line 41 to read:
I will change that for future releases.
Also, in your patch you wrongly spelled "CMAKE_C_COMPILER" as "CMAKE_C_COMPILE".
Alternatively, you may (as "root") type from a terminal:
Then compile the viewer, and revert the changes once done: