Fix expr error in Solaris 10 when using gcc.

[dumol]

The following error is outputted when configuring libffi in Solaris 10 amd64 for GCC:

checking whether .eh_frame section should be read-only... expr: syntax error
no

This introduces a test for a non-empty result when grep'ing the objdump output for eh_frame.

Side note: in Solaris there is no objdump and it seems elfdump -c could be used instead of objdump -h. If that would be case, then the grep command should check for " .eh_frame" instead of eh_frame in conftest.dump, otherwise a second instance of eh_frame will be matched in that file in the string rela.eh_frame.

Please review. Thanks!