Fix build with musl 1.2.4

[kleisauke]

musl 1.2.4 has removed the legacy LFS64 glibc-ABI-compat symbols, which would cause an error during npm install sharp when compiled from source (either with the --build-from-source flag or when a globally installed libvips is detected).

On the API level, the legacy "LFS64" ("large file support") interfaces, which were provided by macros remapping them to their standard names (#define stat64 stat and similar) have been deprecated and are no longer provided under the _GNU_SOURCE feature profile, only under explicit _LARGEFILE64_SOURCE. The latter will also be removed in a future version. Builds broken by this change can be fixed short-term by adding -D_LARGEFILE64_SOURCE to CFLAGS, but should be fixed to use the standard interfaces.
https://musl.libc.org/releases.html

The fix is to replace the OS-specific #ifdef's with a single one that checks whether _LARGEFILE64_SOURCE is defined. Other libc implementations that provide nondegenerate LFS64 interfaces are expected to define this macro as well.

Reproducer

$ mkdir sharp-musl && cd sharp-musl
$ docker run -v $(pwd):/app -w /app -it alpine:edge sh -c "\
    apk add build-base git npm python3 vips-cpp vips-dev && \
    npm install --build-from-source sharp"

...
npm ERR! ../src/pipeline.cc: In member function 'virtual void PipelineWorker::OnOK()':
npm ERR! ../src/pipeline.cc:1267:30: error: aggregate 'PipelineWorker::OnOK()::stat64 st' has incomplete type and cannot be defined
npm ERR!  1267 |         struct STAT64_STRUCT st;
npm ERR!       |                              ^~
npm ERR! ../src/pipeline.cc:1268:55: error: invalid use of incomplete type 'struct PipelineWorker::OnOK()::stat64'
npm ERR!  1268 |         if (STAT64_FUNCTION(baton->fileOut.data(), &st) == 0) {
npm ERR!       |                                                       ^
npm ERR! ../src/pipeline.cc:33:23: note: forward declaration of 'struct PipelineWorker::OnOK()::stat64'
npm ERR!    33 | #define STAT64_STRUCT stat64
npm ERR!       |                       ^~~~~~
npm ERR! ../src/pipeline.cc:1267:16: note: in expansion of macro 'STAT64_STRUCT'
npm ERR!  1267 |         struct STAT64_STRUCT st;
npm ERR!       |                ^~~~~~~~~~~~~
npm ERR! make: *** [sharp-linuxmusl-x64.target.mk:169: Release/obj.target/sharp-linuxmusl-x64/src/pipeline.o] Error 1
...

[lovell]

Dankjewel Kleis!