configure: Fix pkg-config test in absence of valgrind

The empty string used for the not case is replaced by the default
if-else clause and so causes the configure to fail in the absence of
valgrind. Which is not quite what was intended.

Instead use the common idiom of setting a variable depending on whether
the true or false branch is taken and emit the conditional code as a
second step.

Reported-by: Tobias Jakobi <liquid.acid@gmx.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>