Fix next_empty for sequential numbered workspaces.

When all numbered workspaces are sequential (ie, no "holes"), next_empty would
return the last of them instead of the one after the last.

E.g.: When 1, 2 and 3 are the only (numbered) workspaces, next_empty would
return 3, not 4 as intended.
Update PKGBUILD. Fix Makefile.