Indicate failure to find header with blank string

Use empty string instead of zero, so it can be tested with #ifdef as
well as #if in the C++ source code.
Indicate failure to find header with blank string

Use empty string instead of zero, so it can be tested with #ifdef as
well as #if in the C++ source code.
Merge pull request #210 from rolk/210_ifdef

SuiteSparse configuration returns zero instead of blank string when header not found