Hi, thanks for taking the time to write this PR!

It looks like the hdf5 sources are not being built yet, because the make variables need to be named OPTIONAL_WASM_LIBS and OPTIONAL_WASM_BINS to be included in the make all command. Unfortunately, that means the tests have passed but not actually tested your changes.

Once I made that change, some issues have arisen. You should be able to see this yourself by renaming the make variables as suggested, then running:

cd libs; make all

I have made a first pass at correcting the issues, and included the required changes to get building up and running in this review.

I find that the build currently fails with the following:

LD_LIBRARY_PATH="$LD_LIBRARY_PATH`echo -L/Users/georgestagg/work/webr/wasm/lib -s USE_BZIP2=1 -s USE_ZLIB=1 -Oz -fwasm-exceptions -s SUPPORT_LONGJMP=wasm |                  \
                sed -e 's/-L/:/g' -e 's/ //g'`"                               \
         ./H5make_libsettings  H5lib_settings.c  ||                               \
            (test $HDF5_Make_Ignore && echo "*** Error ignored") ||          \
            (rm -f H5lib_settings.c ; exit 1)
/bin/sh: ./H5make_libsettings: Permission denied
make[2]: *** [H5lib_settings.c] Error 1
make[1]: *** [install-recursive] Error 1
emmake: error: 'make install' failed (returned 2)
make: *** [/Users/georgestagg/work/webr/wasm/lib/libhdf5.a] Error 1

Once the changes below have been applied, I will come back later and take another pass at getting hdf5 to compile for Wasm. Possibly H5make_libsettings is an intermediate binary and we just need to patch configure to run it through node, or something similar.

Once the resulting library is compiling and available, it should be that the R package "just works", but we will see.