make distcheck error

[wkliao]

Command make distcheck complains a missing file darshan-core.h.

make -s V=1 LIBTOOLFLAGS=--silent distcheck 
 (cd darshan-runtime && make  top_distdir=../darshan-3.4.0 distdir=../darshan-3.4.0/darshan-runtime \
     am__remove_distdir=: am__skip_length_check=: am__skip_mode_fix=: distdir)
 (cd lib && make  top_distdir=../../darshan-3.4.0 distdir=../../darshan-3.4.0/darshan-runtime/lib \
     am__remove_distdir=: am__skip_length_check=: am__skip_mode_fix=: distdir)
make[6]: *** No rule to make target 'darshan-core.h', needed by 'distdir-am'.  Stop.

[wkliao]

fixed in #843