* 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]
Changes depend on successful find_package(Qt5...) call and when
building against external QScintilla.
Modified FindQScintilla.cmake providing extra hints of locations
to search for needed header files. Hints include paths based on
where Qt5 is installed. Presuming Unix/Linux distributions will
put QScintilla header files under Qt5 header folders.
