Use the (relatively new) macro __has_include() to check if
stdc-predef.h exists, and only include it if it does.
If stdc-predef.h does not exist, or if the compiler does not implement
__has_include(), stdc-predef.h is *not* included.
Fcft no longer uses wchar_t, but plain uint32_t to represent
codepoints.
Since we do a fair amount of string operations in foot, it still makes
sense to use something that actually _is_ a string (or character),
rather than an array of uint32_t.
For this reason, we switch out all wchar_t usage in foot to
char32_t. We also verify, at compile-time, that char32_t used
UTF-32 (which is what fcft expects).
Unfortunately, there are no string functions for char32_t. To avoid
having to re-implement all wcs*() functions, we add a small wrapper
layer of c32*() functions.
These wrapper functions take char32_t arguments, but then simply call
the corresponding wcs*() function.
For this to work, wcs*() must _also_ be UTF-32 compatible. We can
check for the presence of the __STDC_ISO_10646__ macro. If set,
wchar_t is at least 4 bytes and its internal representation is UTF-32.
FreeBSD does *not* define this macro, because its internal wchar_t
representation depends on the current locale. It _does_ use UTF-32
_if_ the current locale is UTF-8.
Since foot enforces UTF-8, we simply need to check if __FreeBSD__ is
defined.
Other fcft API changes:
* fcft_glyph_rasterize() -> fcft_codepoint_rasterize()
* font.space_advance has been removed
* ‘tags’ have been removed from fcft_grapheme_rasterize()
* ‘fcft_log_init()’ removed
* ‘fcft_init()’ and ‘fcft_fini()’ must be explicitly called
