clang-latest now generates -Wextra-semi-stmt in unit tests

[AndrewPaxie]

Daily compile of Trompeloeil unit tests with clang-latest shows this warning generated:

test/compiling_tests_11.cpp:245:24:
warning: empty expression statement has no effect;
remove unnecessary ';' to silence this warning [-Wextra-semi-stmt]
    INFO("msg=" << msg);
                       ^

63 warnings total.

Using Catch version 2.2.3.
Same errors occur with Catch 2.4.2, released 26 October 2018.

Warning was 'refined' in this Clang/LLVM commit

commit 0d4609da69d513b7737d2f932e3db6e57f7b11c3
Author: Roman Lebedev <lebedev.ri@gmail.com>
Date:   Tue Nov 20 18:59:05 2018 +0000

:::

    Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=39111 | PR39111 ]]
    
:::

What to do

Some options:

• Remove the semicolon in Trompeloeil test cases.
• Raise an issue with Catch team asking for a fancier INFO macro.
• Disable the warning in the build.

[AndrewPaxie]

The issue is with Catch2: WARN and CAPTURE macros expect a semicolon
to terminate them (see INTERNAL_CATCH_MSG, INTERNAL_CATCH_CAPTURE).
INFO forwards to INTERNAL_CATCH_INFO which has a trailing semicolon.

The examples in the Catch documentation are also inconsistent, with semicolon
terminating INFO in some examples and no semicolon terminating INFO in
other examples. All examples require a semicolon.

Will raise an issue with the Catch project, if one doesn't already exist.

[AndrewPaxie]

Raised issue in Catch project: catchorg/Catch2#1456.

[rollbear]

Added -Wno-extra-semi-stmt to self_test target, for clang compilers that supports it, so that we can build clean with clang-8. It's a pity, but not a big issue.