doc: add pkg-config requirement for applications

doc: add pkg-config requirement for applications

DPDK relies on pkg-config(1) to provide correct parameters for
compiler and linker used in application build.  Inaccurate build
parameters, produced by pkg-config from DPDK .pc files could fail
application build or cause unpredicted results during application
runtime.

Update system requirements doc about a bug in pkg-config v0.27
used in RHEL-7.

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>