Improve error message for ICU libraries if pkg-config is absent

Hi,

Holger Jacobs complained in pgsql-admin that in v17, if you have the ICU
development libraries installed but not pkg-config, you get a somewhat
unhelpful error message about ICU not being present:

|checking for pkg-config... no
|checking whether to build with ICU support... yes
|checking for icu-uc icu-i18n... no
|configure: error: ICU library not found
|If you have ICU already installed, see config.log for details on the
|failure. It is possible the compiler isn't looking in the proper directory.
|Use --without-icu to disable ICU support.

The attached patch improves things to that:

|checking for pkg-config... no
|checking whether to build with ICU support... yes
|configure: error: ICU library not found
|The ICU library could not be found because pkg-config is not available, see
|config.log for details on the failure. If ICU is installed, the variables
|ICU_CFLAGS and ICU_LIBS can be set explicitly in this case, or use
|--without-icu to disable ICU support.

Michael