Properly set PLDLLIMPEXP_QT and PLDLLIMPEXP_QT_DATA macros for shared library/static devices case
Properly set PLDLLIMPEXP_QT and PLDLLIMPEXP_QT_DATA macros for shared
library/static devices case

This fix solves an issue on Linux where symbols such as vectorize
and QtPLDriver::mutex were not properly exported from libplplot for
the case of a shared library/static devices build.

This fix should solve one of the issues reported at
<https://sourceforge.net/p/plplot/bugs/149/> for the Cygwin platform
and may solve all of them.

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