Fix symbol leak

[phprus]

Fix for issue #3626

@feltech, please test this PR.

[feltech]

Fix looks good to me. I can confirm that, at least for our specific case of GCC 9 and CMake target fmt::fmt-header-only:

Current main leaks symbols:

$ nm --demangle --dynamic --defined-only --extern-only libopenassetio.so | grep fmt
00000000000b55f0 D typeinfo for fmt::v10::format_error
000000000007e410 R typeinfo name for fmt::v10::format_error
$

and this PR does not ;)

$ nm --demangle --dynamic --defined-only --extern-only libopenassetio.so | grep fmt
$

I'm still personally curious why this special case for format_error exists (not that its wrong, I just genuinely want to understand), but I'll follow-up in the issue.

[vitaut]

Thanks for the PR.

[vitaut]

I don't think we should introduce a new macro. Let's update FMT_VISIBILITY instead since we shouldn't be changing visibility in general when not compiling a shared library.

[phprus]

@vitaut
FMT_VISIBILITY is used unconditionally:

fmt/include/fmt/format.h

Lines 1883 to 1885 in a4b7b24

	/* Use the hidden visibility as a workaround for a GCC bug (#1973). */ \
	/* Use a macro-like name to avoid shadowing warnings. */ \
	struct FMT_VISIBILITY("hidden") FMT_COMPILE_STRING : base { \

Therefore, we need two macros.

[vitaut]

OK, we can simplify this later.

[vitaut]

Merged, thanks!