Ensure qRestAPI_Export.h is available for project depending on qRestAPI

Following commit 6339e0, "qRestAPI_Export.h.in" is configured into
"qRestAPI_Export.h" which is located in the build tree.

This commit ensure that the folder containing the configured file
is added to the list of directory to be included by project calling
"find_package(qRestAPI [...])"
Define qRestAPI_EXPORT on Unix platform

While commit 6339e0cc fixed windows static build it also broke unix one.
This commit fixes the problem.
Merge branch 'support-windows-static-build'

* support-windows-static-build:
  Define qRestAPI_EXPORT on Unix platform
  Ensure qRestAPI_Export.h is available for project depending on qRestAPI