gnucash-docs maint: Define ac_abs_top_srcdir before first use

[Frank H.Ellenberger]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/052d88c0 (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/ab991372 (commit)



commit 052d88c001408ad630c2a8ae266b1b581b223c41
Author: Frank H. Ellenberger <frank.h.ellenberger at gmail.com>
Date:   Wed Aug 7 01:45:03 2019 +0200

    Define ac_abs_top_srcdir before first use
    
    Supplement of commit ab99137



Summary of changes:
 configure.ac | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)