Improve configure error for ICU libraries if pkg-config is absent.

Improve configure error for ICU libraries if pkg-config is absent.

If pkg-config is not installed, the ICU libraries cannot be found, but
the custom configure error message did not mention this. This might
lead to confusion about the actual problem. To improve this, remove
the explicit error message and rely on PKG_CHECK_MODULES' generic
error message.

Author: Michael Banck
Reported-by: Holger Jakobs
Discussion: https://postgr.es/m/ccd579ed-4949-d3de-ab13-9e6456fd2caf%40jakobs.com
Discussion: https://postgr.es/m/66b5d05c.050a0220.7c8ce.a951@mx.google.com