Fix build on musl

[ibc]

Build of applications using flatbuffers such as mediasoup (see related issue https://github.com/versatica/mediasoup/issues/1223) are broken on musl (such as in Alpine Linux) since version 1.11.0 and google@5f32f94 because strtoll_l (and strtoull_l) are not available on musl.

flatbuffers checks for the availability of strtoull_l in CMakeLists.txt so flatbuffers builds successfully, but for applications using flatbuffers the result of this check is not available and FLATBUFFERS_LOCALE_INDEPENDENT is set to 1 resulting in the following build failure:

In file included from ../../../subprojects/flatbuffers-23.3.3/include/flatbuffers/minireflect.h:21,
  from ../../../include/Channel/ChannelRequest.hpp:8,
  from ../../../include/Settings.hpp:6,
  from ../../../include/Logger.hpp:91,
  from ../../../src/RTC/RTCP/XrReceiverReferenceTime.cpp:5:
  ../../../subprojects/flatbuffers-23.3.3/include/flatbuffers/util.h: In function 'void flatbuffers::strtoval_impl(int64_t, const char*, char**, int)':
  ../../../subprojects/flatbuffers-23.3.3/include/flatbuffers/util.h:229:38: error: 'strtoll_l' was not declared in this scope; did you mean 'strtold_l'?
  229 | #define __strtoll_impl(s, pe, b) strtoll_l(s, pe, b, ClassicLocale::Get())
      |         ^~~~~~~~~~~~~~

This issue was reported in https://github.com/google/flatbuffers/pull/6773 but was closed without a solution. Then a flatbuffers fork made this exact commit: https://github.com/sartura/flatbuffers/commit/92bd62407329caacd66e92e5bfd2949f2f137bfe

Due to rationale given in the closed issue, I'm not sure this PR will be accepted. If not, I strongly think that flatbuffers documentation should say that applications using it in musl environments must define FLATBUFFERS_LOCALE_INDEPENDENT=0 in their main project.

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[github-actions[bot]]

This pull request is stale because it has been open 6 months with no activity. Please comment or label not-stale, or this will be closed in 14 days.

[ibc]

not-stale

[ibc]

I've signed the Google Individual Contributor License Agreement.

[ibc]

@dbaileychess @aardappel (just mentioning some maintainers randomly), could you please approve CI workflows of this PR?