OpenBSD: Fix system feature definitions

Since commit f034b0f6 (CMake compilation: do not use compiler
extensions, 2020-03-14, v3.18.0-rc1~494^2), some sources explicitly
enable needed system APIs on some platforms using definitions like
`_POSIX_C_SOURCE` and `_XOPEN_SOURCE`.  Drop the definitions for
OpenBSD, which provides the POSIX APIs by default.
25 jobs for !6000 with fix/openbsd-support in 6 minutes and 6 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Build
manual build:centos6-x86_64 #5703316
cmake build docker linux manual
passed build:debian10-iwyu #5703313
cmake build docker linux

00:04:01

manual build:fedora33-ninja #5703317
cmake build docker linux linux-3.17 manual
passed build:fedora33-sphinx #5703315
cmake build docker linux linux-3.17

00:03:31

passed build:fedora33-tidy #5703314
cmake build docker linux linux-3.17

00:06:05

manual build:macos-x86_64-makefiles #5703319
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:macos-x86_64-ninja #5703318
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #5703320
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent manual
 
  Test
created test:fedora33-ninja #5703321
cmake docker linux linux-3.17 x11
created test:macos-x86_64-makefiles #5703323
cmake macos shell xcode-12.4 nonconcurrent
created test:macos-x86_64-ninja #5703322
cmake macos shell xcode-12.4 nonconcurrent
created test:windows-vs2019-x64-ninja #5703324
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent
 
  Test Ext
created test:cuda10.2-nvidia #5703327
cmake cuda-rt docker linux
created test:debian10-ninja #5703325
cmake build docker linux linux-3.17
created test:fedora33-makefiles #5703326
cmake build docker linux linux-3.17
created test:fedora33-ninja-multi #5703328
cmake build docker linux linux-3.17
created test:macos-x86_64-xcode #5703329
cmake macos shell xcode-12.4 concurrent
created test:windows-vs2019-x64 #5703330
cmake windows shell vs2019 msvc-19.28-16.9 concurrent
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash