configure: do not use bash's expr match

It looks like there are some incompatibilities in bash's expr match
among different versions. Using set -x, gives us on bash 4.2.45:

+ TOOLS=' netsniff-ng trafgen ifpps bpfc'
+ for tool in '$_tools'
+ case "$tool" in
++ expr match '"curvetun' 'astraceroute"' '$1'
expr: syntax error
+ m=
+ '[' '' == 0 ']'

So rather avoid using "expr match" and switch to "case" for substring
matching, which should be more portable.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>