Merge #1722: docs: Exclude modules' bench_impl.h headers from coverage report

0458def doc: Add `--gcov-ignore-parse-errors=all` option to `gcovr` invocations (Hennadii Stepanov)
1aecce5 doc: Add `--merge-mode-functions=separate` option to `gcovr` invocations (Hennadii Stepanov)
106a7cb doc: Exclude modules' `bench_impl.h` headers from coverage report (Hennadii Stepanov)
a9e955d autotools, docs: Adjust help string for `--enable-coverage` option (Hennadii Stepanov)

Pull request description:

  Additionally, [this](#1113 (comment)) comment has been addressed.

ACKs for top commit:
  real-or-random:
    utACK 0458def
  josibake:
    ACK 0458def

Tree-SHA512: 23306435fa581bb492df92427bbb8dea1b6edabd71a79acd2e2c142af599639690e6b01e2453a0700a38ec78997bfc2b0f38b7cb0d5e5af9db12dd4cea271f34

Author: real-or-random

CONTRIBUTING.md

@@ -92,12 +92,14 @@ Run the tests:
 
 To create a report, `gcovr` is recommended, as it includes branch coverage reporting:
 
-    $ gcovr --exclude 'src/bench*' --print-summary
+    $ gcovr --gcov-ignore-parse-errors=all --merge-mode-functions=separate --exclude 'src/bench*' --exclude 'src/modules/.*/bench_impl.h' --print-summary
 
 To create a HTML report with coloured and annotated source code:
 
     $ mkdir -p coverage
-    $ gcovr --exclude 'src/bench*' --html --html-details -o coverage/coverage.html
+    $ gcovr --gcov-ignore-parse-errors=all --merge-mode-functions=separate --exclude 'src/bench*' --exclude 'src/modules/.*/bench_impl.h' --html --html-details -o coverage/coverage.html
+
+On `gcovr` >=8.3, `--gcov-ignore-parse-errors=all` can be replaced with `--gcov-suspicious-hits-threshold=140737488355330`.
 
 #### Exhaustive tests
 

configure.ac

@@ -144,7 +144,7 @@ AC_ARG_ENABLE(benchmark,
     [SECP_SET_DEFAULT([enable_benchmark], [yes], [yes])])
 
 AC_ARG_ENABLE(coverage,
-    AS_HELP_STRING([--enable-coverage],[enable compiler flags to support kcov coverage analysis [default=no]]), [],
+    AS_HELP_STRING([--enable-coverage],[enable coverage analysis support [default=no]]), [],
     [SECP_SET_DEFAULT([enable_coverage], [no], [no])])
 
 AC_ARG_ENABLE(tests,