Some machines, like LLNL udawn, a blue-gene machine, requires all executables,
be launched by some command like mpirun.
Solution:
Added $RUNSERIAL to launch the executable.
Tested: LLNL uDawn.
1) Move the test for H5D_EXT_PREFIX in links.c to a separate file: links_env.c
2) links_env.c will be used by testlinks_env.sh to test for the environmental
variable H5D_EXT_PREFIX in searching for the external linked file.