Don't reinvent find_package

PythonInterp is a finder module that ships with cmake. It supports
the conventional interaction with find_package that allows you
to demand success, and particular vesions, without having your
own logic:

https://cmake.org/cmake/help/v3.0/command/find_package.html

hipify-clang/CMakeLists.txt

@@ -66,14 +66,7 @@ install(TARGETS hipify-clang DESTINATION bin)
 
 if (HIPIFY_CLANG_TESTS)
     # tests
-    set(Python_ADDITIONAL_VERSIONS 2.7)
-    include(FindPythonInterp)
-    if(NOT PYTHONINTERP_FOUND)
-        message(FATAL_ERROR
-            "Unable to find Python interpreter, required for builds and testing.\n\n"
-            "Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
-    endif()
-
+    find_package(PythonInterp 2.7 REQUIRED EXACT)
     find_program(LIT_COMMAND lit)
 
     set(BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})