configure/CMakelists: Use Python_EXECUTABLE as hint for interpreter

The PYTHON_EXECUTABLE variable was only interpreted by our own FindPythonDev.
cmake's FindPython uses Python_EXECUTABLE instead.

Also, if --with-python is not used, prefer unversioned executables that
match the version requirement. Further, remove Python_ADDITIONAL_VERSIONS
that is interpreted by FindPythonInterp which we're not using anymore.

https://cmake.org/cmake/help/latest/module/FindPython.html#artifacts-specification
https://cmake.org/cmake/help/latest/module/FindPythonInterp.html

Closes #3275

CMakeLists.txt

@@ -735,8 +735,8 @@ if (NOT SED_EXE)
     endif ()
 endif ()
 
-list(APPEND Python_ADDITIONAL_VERSIONS 3)
 set(ZEEK_PYTHON_MIN 3.5.0)
+set(Python_FIND_UNVERSIONED_NAMES FIRST)
 find_package(Python ${ZEEK_PYTHON_MIN} REQUIRED COMPONENTS Interpreter)
 find_package(FLEX REQUIRED)
 find_package(BISON 2.5 REQUIRED)

configure

@@ -365,7 +365,7 @@ while [ $# -ne 0 ]; do
             append_cache_entry OPENSSL_ROOT_DIR PATH $optarg
             ;;
         --with-python=*)
-            append_cache_entry PYTHON_EXECUTABLE PATH $optarg
+            append_cache_entry Python_EXECUTABLE PATH $optarg
             ;;
         --with-python-inc=*)
             append_cache_entry PYTHON_INCLUDE_DIR PATH $optarg