C library: getrandom requires glibc >= 2.25

[tautschnig]

As noticed in Homebrew/homebrew-core#102337,
which attempts to build on Ubuntu 16.04, sys/random.h isn't available
with glibc versions prior to 2.25. Therefore, replace the linux
inclusion check by one that tests for a particular glibc version.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6889 (df444b7) into develop (9af369f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6889   +/-   ##
========================================
  Coverage    77.80%   77.80%           
========================================
  Files         1567     1567           
  Lines       179920   179920           
========================================
  Hits        139992   139992           
  Misses       39928    39928           

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