configure: set platform variables also when uname is unrecognized

Since commit

124a67e96e: configure: add set -u

all variables must be set before their expansion are attempted. These
2 variables: "platform" and "linker_resolves_library_dependencies" were
not given value in the final 'else' branch when platform check failed
due to unrecognized kernel name (output of `uname`). Now those two are
given reasonable non-empty values.

configure

@@ -650,6 +650,8 @@ elif [ $uname = "Linux" ] || [ $uname = "GNU" ] ; then
     fi
 else
     printf "Unknown.\n"
+    platform="$uname"
+    linker_resolves_library_dependencies=0
     cat <<EOF
 
 *** Warning: Unknown platform. Notmuch might or might not build correctly.