We previously defined `_POSIX_C_SOURCE` and friends while building
binary packages in order to minimize the version of glibc needed at
runtime.  The definitions were added by commit facc240a45
(Utilities/Release: Add docker specs to build and test Linux binaries,
2019-08-23, v3.16.0-rc1~184^2~2), but came from older packaging scripts
that were removed by commit 689fdbfc61 (Utilities/Release: Drop linux64
script in favor of docker build, 2019-08-27, v3.16.0-rc1~184^2).  Those
older scripts were meant for use in a hand-maintained environment.  Now
that we use base images of old CentOS versions to establish the build
environment, our builds are already limited to older glibc versions
(glibc 2.12 from centos6 on x86_64, and 2.17 from centos7 on aarch64).

Our old system API definitions no longer affect the glibc version
required by the binaries.  Drop them to avoid potential conflicts with
system API definitions added by changes like commit f034b0f663 (CMake
compilation: do not use compiler extensions, 2020-03-14, v3.18.0-rc1~494^2)
and commit c7c3e39e4f (Utilities: Activate POSIX APIs even without
compiler extensions, 2022-06-02).