* Enable build against external QCustomPlot
Source-base distribution has stand-alone QCustomPlot library package.
Feature request is to modify the build instructions to enable use of
external library, rather than building internal supplied library,
with cmd-line switch.
If unable to locate external library, fallback to building internal QCustomPlot.
Build internal is on by default.
* Enable build against external QCustomPlot #1784 - revised
Invert naming/logic:
FORCE_INTERNAL_QCUSTOMPLOT=ON(default) will build QCustomPlot library provided sources
(consistent with current processes)
FORCE_INTERNAL_QCUSTOMPLOT=OFF will search for external QCustomPlot library on system and
fall back to internal if not found.
[github #1784]
