Small bug in grep 2.5.3 on AIX (probably for all platforms)

[Perry Smith]

The Makefile in the po directory tries to use mkdir_p (lowercase).  It  
is set to:

mkdir_p = $(top_builddir)/./install-sh -c -d

but top_builddir is not set.  There is a variable called top_srcdir.   
Perhaps that is one that is intended.

Thank you,
Perry