don't set python exe to undefined

Summary:
FindPythonInterp sets PYTHON_EXECUTABLE, we overrode it by setting an
empty var. add a comment to explain the unbalancedness of the conditional
branches.

Test Plan: pythoncairo is correctly found with cmake 3.10.2

Reviewers: davidedmundson, fvogt

Reviewed By: fvogt

Subscribers: fvogt, heikobecker, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18309

cmake/FindPythonCairo.cmake

@@ -1,6 +1,6 @@
 if(CMAKE_VERSION VERSION_LESS 3.12.0)
     find_package(PythonInterp 3 REQUIRED)
-    set(PYTHON_EXECUTABLE "${Python3_EXECUTABLE}")
+    # PythonInterp sets PYTHON_EXECUTABLE
 else()
     find_package(Python3 COMPONENTS Interpreter REQUIRED)
     set(PYTHON_EXECUTABLE "${Python3_EXECUTABLE}")