Improve condition for declaring struct timeval

[mjbogusz]

Summary of changes

Improve condition for declaring struct timeval - do it only if it wasn't declared before.

This fixes usage of clang-tidy.

Without this fix, I'm hitting errors like this when using clang-tidy (paths slightly redacted):

/path-to-my-code/mbed-os/platform/include/platform/mbed_rtc_time.h:37:8: error: redefinition of 'timeval' [clang-diagnostic-error]
struct timeval {
       ^
/opt/gcc-arm-none-eabi/arm-none-eabi/include/sys/_timeval.h:54:8: note: previous definition is here
struct timeval {
       ^

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@mjbogusz, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Why are we removing __clang__ ?

[mjbogusz]

Because it was causing the double-defined errors when using clang-tidy.
AFAIK clang is not supported as a compiler for Mbed anyway, so it shouldn't break anything, but this change allows usage of clang-tidy on a compilation database generated for the GCC toolchain.

I've tried to check for defines used in the headers declaring the struct, but there is no consistency between them, so the alternative solution would be to cover all the known define flags and decide whether do define struct timeval based on that.
The defines I've found so far are:

• The libc and derivatives (e.g. Ubuntu/Debian package libc6-dev) provides #define __timeval_defined 1, e.g. in /usr/include/<architecture_specific>/bits/types/struct_timeval.h
• The newlib (e.g. package libnewlib-dev) provides #define _TIMEVAL_DEFINED, e.g. in /usr/include/newlib/sys/_timeval.h
  I don't have ARM's microlib at hand to inspect what defines it does provide.

[0xc0170]

if we define our own, and check if not yet defined (_TIMEVAL_DEFINED), we should define it.
adding on line 39 #define _TIMEVAL_DEFINED so anyone getting this header, can check if its defined and it is (by us)

[0xc0170]

we define gnu flag for compatibility for ARMClang, so __GNUC__ is defined. I assume ARMClang do not have defined _TIMEVAL_DEFINED (seeing similar implementation https://github.com/cesanta/mjs/blob/master/mjs.c#L1054 for mbed).

I'll fetch this to test with ARMClang, I cant quickly find in the online docs.

[mjbogusz]

Maybe the most compatible solution would be to drop the defined(__GNUC__) check whatsoever and decide only based on the existance of the _TIMEVAL_DEFINED (and its variants)?

[0xc0170]

I think so then. I'll check if we use in tests timeval so our CI tests this with both toolchains we support.

[mjbogusz]

I've pushed a version with checking for the defines - let me know if there are any more conditions we should check.

[0xc0170]

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