cmake: install JpegWriter.h depending on libjpeg

... and not libopenjpeg
qt4/qt5: improve naming of internal export/import macros

use something less generic than LIB_EXPORT/LIB_IMPORT
qt4/qt5: add GCC visibility export attributes

this does no actual changes to the exported symbols, but can help in
hiding symbols if the right GCC flags for symbols visibility are
specified
Use c99 for the c compiler
[qt] Improve compilation under Win 8 with Visual Studio 2012

Bug #73111