Opt-out of std::locale via cmake -DENABLE_LOCALE=N

[nigels-com]

Thanks for muparser, it's getting the job done nicely.

I don't have at hand the details of why we needed to opt-out of std::locale, but this is the main divergence we have.

[beltoforion]

I'm sorry but "I don't know why we need this" is not a compelling reason for a merge. I don't see a use in the change and it is altering the API by removing the following functions:

parser.SetDecSep(',');
parser.SetThousandsSep('.');

[nigels-com]

Fair enough! A somewhat less vague rationale, looking at some slack threads from a year ago.

A colleague was having a crash at shutdown on Ubuntu 20.04 (most likely). valgrind suggested that was to do with static object order of construction and destruction. In particular: mu::ParserBase::s_locale

Because it's static and opaque std::locale the fix (or workaround perhaps) is this change to opt-out of locale support.

[nigels-com]

Whether that's technically a bug in the relevant plugin architecture, dlopen stuff, or somehow because we're statically linking muparser, not clear. I'd be inclined to avoid having static std::locale (ideally).

[nigels-com]

A somewhat redacted valgrind fragment:

==121635== Invalid read of size 4
==121635==    at 0x4D9AB05: std::locale::~locale() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.28)
==121635==    by 0x50728A6: __run_exit_handlers (exit.c:108)
==121635==    by 0x5072A5F: exit (exit.c:139)
==121635==    by 0x5050089: (below main) (libc-start.c:342)
==121635==  Address 0x54905d0 is 0 bytes inside a block of size 40 free'd
==121635==    at 0x4A3BE99: operator delete(void*) (vg_replace_malloc.c:923)
==121635==    by 0x50728A6: __run_exit_handlers (exit.c:108)
==121635==    by 0x5072A5F: exit (exit.c:139)
==121635==    by 0x5050089: (below main) (libc-start.c:342)
==121635==  Block was alloc'd at
==121635==    at 0x4A397B3: operator new(unsigned long) (vg_replace_malloc.c:422)
==121635==    by 0x2AF3C0: std::locale::locale<mu::ParserBase::change_dec_sep<char> >(std::locale const&, mu::ParserBase::change_dec_sep<char>*) (in ...)
==121635==    by 0x167C586D: _GLOBAL__sub_I_muParserBase.cpp (in ... /fooplugin.so)
==121635==    by 0x4011B99: call_init.part.0 (dl-init.c:72)
==121635==    by 0x4011CA0: call_init (dl-init.c:30)
==121635==    by 0x4011CA0: _dl_init (dl-init.c:119)
==121635==    by 0x518C984: _dl_catch_exception (dl-error-skeleton.c:182)
==121635==    by 0x40160CE: dl_open_worker (dl-open.c:758)
==121635==    by 0x518C927: _dl_catch_exception (dl-error-skeleton.c:208)
==121635==    by 0x4015609: _dl_open (dl-open.c:837)
==121635==    by 0x545E34B: dlopen_doit (dlopen.c:66)
==121635==    by 0x518C927: _dl_catch_exception (dl-error-skeleton.c:208)
==121635==    by 0x518C9F2: _dl_catch_error (dl-error-skeleton.c:227)
==121635==    by 0x545EB58: _dlerror_run (dlerror.c:170)
==121635==    by 0x545E3D9: dlopen@@GLIBC_2.2.5 (dlopen.c:87