Fix check-local failure when quilt patches are applied

[Bas Couwenberg]

The check-local target failed because it has the spelling error quilt patch applied:

make  check-local
make[5]: Entering directory '/build/geos-3.7.0~alpha'
! find . -name '*.cpp' -o -name '*.h' | \
        grep -v ^./tests/xmltester/tinyxml | \
        xargs grep -n '[[:space:]]$'
./.pc/spelling-errors.patch/tests/xmltester/tinyxml/tinyxml.h:83:#endif
./.pc/spelling-errors.patch/tests/xmltester/tinyxml/tinyxml.h:98:/*     Internal structure for tracking location of items
...
./.pc/spelling-errors.patch/tests/xmltester/tinyxml/tinyxml.h:1764:     /** Switch over to "stream printing" which is the most dense formatting without
./.pc/spelling-errors.patch/tests/xmltester/tinyxml/tinyxml.h:1769:                                                                                                     }
make[5]: *** [Makefile:931: check-local] Error 1
make[5]: Leaving directory '/build/geos-3.7.0~alpha'

The attached patch fixes the issue by using a less strict pattern to exclude.

[Sandro Santilli <strk@…>]

In 9eb8152/git:

Better skip external sources from checking blanks

Patch by Bas Couwenberg
Closes #888