oksh-6.8.1

This is oksh-6.8 with the following additions:

• Fix build on musl-libc platforms (#49)
• Default CFLAGS of -g -O2 if user didn't specify any
• Fix a bug on Linux systems (#48)
• Add an OKSH_VERSION variable that gives the user the oksh version (KSH_VERSION and SH_VERSION are still their original values)

Please use the versioned tarball!