libclc: Use find_package to find Python 3 and require it

[aaronpuchert]

Summary

The script's shebang wants Python 3, so we use FindPython3. The
original code didn't work when an unversioned python was not available.
This is explicitly allowed in PEP 394. ("Distributors may choose to set
the behavior of the python command as follows: python2, python3, not
provide python command, allow python to be configurable by an end user
or a system administrator.")

Also I think it's actually required, so let the configuration fail if we
can't find it.

Lastly remove the shebang, since the script is only run via interpreter
and doesn't have the executable bit set anyway.

[jvesely]

I think this should use FindPython3 and drop the shebang from gen_convert.py to fix the 2 vs. 3 issues as well.

[aaronpuchert]

Use FindPython3 and remove shebang. The script didn't have an executable bit anyway.

[jvesely]

LGTM

libclc/CMakeLists.txt

if( ENABLE_RUNTIME_SUBNORMAL )
   add_library( subnormal_use_default STATIC
     generic/lib/subnormal_use_default.ll )
   add_library( subnormal_disable STATIC
     generic/lib/subnormal_disable.ll )
   install( TARGETS subnormal_use_default subnormal_disable ARCHIVE
     DESTINATION ${CMAKE_INSTALL_DATADIR}/clc )
 endif()
 
-find_program( PYTHON python )
+find_package( Python REQUIRED COMPONENTS Interpreter )
 file( TO_CMAKE_PATH ${CMAKE_SOURCE_DIR}/generic/lib/gen_convert.py script_loc )
 add_custom_command(
   OUTPUT convert.cl
-  COMMAND ${PYTHON} ${script_loc} > convert.cl
+  COMMAND ${Python_EXECUTABLE} ${script_loc} > convert.cl
   DEPENDS ${script_loc} )
 add_custom_target( "generate_convert.cl" DEPENDS convert.cl )
 
 enable_testing()
 
 foreach( t ${LIBCLC_TARGETS_TO_BUILD} )
   message( "BUILDING ${t}" )
   string( REPLACE "-" ";" TRIPLE  ${t} )