make uncompiled exported headers compilable

(shamelessly stolen inspired by https://gitlab.cern.ch/clemenci/Gaudi/commits/GAUDI-1231.add-test-build-of-all-headers)

The idea is that any public header "GaudiComponent/UseMe.h" should be usable in a oneline "UseMe.cpp" that includes the header and nothing else. (i.e. anding an include of a Gaudi header should not change a file from compilable to non-compilable).

• [2018-01-25 21:42] Validation started with lhcb-gaudi-merge#395
• [2018-01-26 00:05] Validation started with lhcb-gaudi-head-py3#45
• [2018-01-26 00:05] Validation started with lhcb-tdr-test#38
• [2018-01-26 00:07] Validation started with lhcb-gaudi-head#1735
• [2018-01-26 08:43] Validation started with lhcb-gaudi-head#1736
• [2018-01-27 00:04] Validation started with lhcb-gaudi-head-py3#46
• [2018-01-27 00:06] Validation started with lhcb-tdr-test#39
• [2018-01-27 00:08] Validation started with lhcb-gaudi-head#1737
• [2018-01-28 00:05] Validation started with lhcb-gaudi-head-py3#47
• [2018-01-28 00:06] Validation started with lhcb-tdr-test#40
• [2018-01-28 00:08] Validation started with lhcb-gaudi-head#1738
• [2018-01-29 00:04] Validation started with lhcb-gaudi-head-py3#48
• [2018-01-29 00:05] Validation started with lhcb-tdr-test#41
• [2018-01-29 00:09] Validation started with lhcb-gaudi-head#1739

added 1 commit

• d6856a11 - clang format (hope version compatible enough with CI)

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mentioned in merge request !592 (merged)

approved this merge request

changed milestone to %v30r2

assigned to @hegner

resolved all discussions

merged

mentioned in commit 6da48645

added bug fix label