Use Boost filesystem instead of std::fs with clang 10

This is a workaround for the fact that the clang10 build of the LCG stack was using gcc 9.2 libstdc++, which results in a link error for debug builds when using libstdc++fs.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91067 and LBCORE-1923.

/cc @chasse

changed milestone to %v34r1

added lhcb-gaudi-head lhcb-lcg-dev3 lhcb-lcg-dev4 labels

changed the description

• [2020-09-09 00:08] Validation started with lhcb-lcg-dev4#1398
• [2020-09-09 00:11] Validation started with lhcb-lcg-dev3#1383
• [2020-09-09 00:17] Validation started with lhcb-gaudi-head#2714
• [2020-09-09 00:21] Validation started with lhcb-gaudi-head-py3#465
• [2020-09-10 00:04] Validation started with lhcb-lcg-dev4#1399
• [2020-09-10 00:08] Validation started with lhcb-lcg-dev3#1384
• [2020-09-10 00:12] Validation started with lhcb-gaudi-head-py3#466
• [2020-09-10 00:36] Validation started with lhcb-gaudi-head#2715
• [2020-09-11 00:09] Validation started with lhcb-lcg-dev4#1400
• [2020-09-11 00:09] Validation started with lhcb-lcg-dev3#1385
• [2020-09-11 00:20] Validation started with lhcb-gaudi-head#2716
• [2020-09-11 00:26] Validation started with lhcb-gaudi-head-py3#467
• [2020-09-12 00:08] Validation started with lhcb-lcg-dev3#1386
• [2020-09-12 00:09] Validation started with lhcb-lcg-dev4#1401
• [2020-09-12 00:19] Validation started with lhcb-gaudi-head#2717
• [2020-09-13 00:03] Validation started with lhcb-lcg-dev4#1402
• [2020-09-13 00:03] Validation started with lhcb-lcg-dev3#1387
• [2020-09-14 00:07] Validation started with lhcb-lcg-dev4#1403
• [2020-09-14 00:08] Validation started with lhcb-lcg-dev3#1388
• [2020-09-14 00:16] Validation started with lhcb-gaudi-head#2718

mentioned in commit 9b101dac

merged

mentioned in issue lhcb/Allen#229 (closed)