Update to version 1.12.21.

Pkgsrc changes:
* Modify license to dual either gpl2 or gpl3.
* Adjust BUILDLINK_API_DEPENDS settings from configure.ac.
* Adjust PLIST to files now installed.
* Add comments to the existing patches.

Upstream notable changes (see ChangeLog in source for details):
gnumeric 1.12.21:
  * ODS import/export fixes
  * XLSX import/export fixes

gnumeric 1.12.20:
  * ODF import/export fixes and enhancements.
  * XLSX import/export fixes and enhancements.

gnumeric 1.12.19:
  * (no noteworthy major changes)

gnumeric 1.12.18:
  * Workaround gtk+ api/abi breaks.

gnumeric 1.12.17:
  * xlsx import/export improvements.

gnumeric 1.12.16:
  * ODS import/export improvements

gnumeric 1.12.15:
  * ods/xlsx graph import fixes
  * ods import/export fixes

gnumeric 1.12.14:
  * Many fixes related to conditional formats.
  * Try to disable Ubuntu's non-working scroll bars.

gnumeric 1.12.13:
  * ods import/export export fixes.
  * xls import/export export fixes.

gnumeric 1.12.12:
  * Test suite improvements.
  * ods import/export improvements
  * xls import/export improvements
  * xlsx import/export improvements

gnumeric 1.12.11:
  * A problem with saving to Excel97 format was fixed.

gnumeric 1.12.10:
  * Accuracy improvements
  * Work around gtk+ abi breaks
  * Win32 improvements

(he)