add support to include files with BOM character

[efferre79]

some upstream opencascade files include a BOM character at the beginning which leads to an error when parsing the file by clang

This will make out-of-date two other OCP PRs https://github.com/CadQuery/OCP/pull/134 and https://github.com/CadQuery/OCP/pull/135. The proposed solution here doesn't require to patch the opencascade include files which might come also from the user system.

[efferre79]

this fixes errors such as:

[W 240324 22:27:18 translation_unit:58] dummy.cxx:12:1: error: unexpected character <U+FEFF>

during OCP parsing of headers