build: fix some build issues encountered on a musl libc system (AcademySoftwareFoundation#4903)

Two small fixes to  compile on musl libc systems.

The first commit is about the use of `__WORDSIZE`, which is not a
standard define and is defined another header in musl libc. I decided to
fix this by following what was done in `libdnf` in [this
commit](rpm-software-management/libdnf@82a07db).
However, there are other ways, such as including `bits/reg.h` or
`bits/user.h`, which define `__WORDSIZE` in musl libc, or using
`__LP64__` to check for 64 bit instead (inspired by [Alpine Linux's
patch](https://gitlab.alpinelinux.org/alpine/aports/-/blob/0f71e9504b59049e0ec1abb486ba455ab2650c83/community/openimageio/0001-fix-compile-error.patch)):

```c
#if defined(__GNUC__) && (__WORDSIZE == 64 || (defined(__LP64__) && __LP64__)) && !(defined(__APPLE__) && defined(__MACH__)) || defined(__NetBSD__)
```

The second commit is about the use of the `feenableexcept` function.
This function is only defined by glibc. The original code assumed all
Linux systems use glibc (checking `__linux__` define), so I just changed
it to check for glibc directly (by checking the `__GLIBC__` define).


I have only tested it on my musl libc system. I think just compiling it
on different systems is enough of a test in this case...

---------

Signed-off-by: omcaif <[email protected]>

Author: omcaif

src/include/OpenImageIO/typedesc.h

@@ -409,7 +409,7 @@ template<> struct BaseTypeFromC<uint64_t> { static constexpr TypeDesc::BASETYPE
 template<> struct BaseTypeFromC<const uint64_t> { static constexpr TypeDesc::BASETYPE value = TypeDesc::UINT64; };
 template<> struct BaseTypeFromC<int64_t> { static constexpr TypeDesc::BASETYPE value = TypeDesc::INT64; };
 template<> struct BaseTypeFromC<const int64_t> { static constexpr TypeDesc::BASETYPE value = TypeDesc::INT64; };
-#if defined(__GNUC__) && __WORDSIZE == 64 && !(defined(__APPLE__) && defined(__MACH__)) || defined(__NetBSD__)
+#if defined(__GNUC__) && (ULONG_MAX == 0xffffffffffffffff) && !(defined(__APPLE__) && defined(__MACH__)) || defined(__NetBSD__)
 // Some platforms consider int64_t and long long to be different types, even
 // though they are actually the same size.
 static_assert(!std::is_same_v<unsigned long long, uint64_t>);

src/libOpenImageIO/imageinout_test.cpp

@@ -537,7 +537,7 @@ main(int argc, char* argv[])
     getargs(argc, argv);
 
     if (enable_fpe) {
-#if defined(__linux__)
+#if defined(__GLIBC__)
         fprintf(stderr, "Enable floating point exceptions.\n");
         feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
 #else