Support read-only source

If the sources for zstd is located on a read-only filesystem (readonly NFS for instance), even with BUILD_DIR set to a writeable directory, the build will fail:

make: Entering directory '/usr/src/zstd-1.5.5/lib'
Installing includes
make[1]: Entering directory '/usr/src/zstd-1.5.5/lib'
CC /tmp/zstd/dynamic/entropy_common.o
CC /tmp/zstd/dynamic/xxhash.o
CC /tmp/zstd/dynamic/zstd_compress_sequences.o
compiling multi-threaded dynamic library 1.5.5
creating versioned links
ln: failed to create symbolic link 'libzstd.so.1': Read-only file system
make[1]: *** [Makefile:163: /tmp/zstd/dynamic/libzstd.so.1.5.5] Error 1
make[1]: Leaving directory '/usr/src/zstd-1.5.5/lib'
make: *** [Makefile:330: install-shared] Error 2
make: Leaving directory '/usr/src/zstd-1.5.5/lib'