Could you check if you have ./ldap/admin/src/scripts/dbmon.sh in your build tree? I'm suspecting you might have wiped out by some clean procedure.
The file dbmon.sh is removed after make clean because it is added to AUTOMAKE variable CLEANFILES.
Details http://www.gnu.org/software/automake/manual/html_node/Clean.html
BTW "make clean" removes also other files from git tree deleted: ldap/admin/src/scripts/dbmon.sh deleted: ldap/ldif/50posix-winsync-plugin.ldif deleted: ldap/ldif/50replication-plugins.ldif deleted: ldap/ldif/90betxn-plugins.ldif
I expect that these files should not be removed with make clean.
Must have been when I did a make clean earlier.
I have now run a git reset --hard HEAD to restore the files, and the build appears to be working.