QMake does not define _WINDLL when generating DLL makefiles on Windows

Visual Studio automatically defines _WINDLL when building a DLL, regardless of project settings (https://msdn.microsoft.com/en-us/library/8x480de8.aspx). This define is therefore widely used to detect DLL vs. static library or executable build target on Windows.

VS project files generated by QMake does correctly define _WINDLL. However, _WINDLL is not defined for QMake generated makefiles for DLL projects on Windows. This causes problems for my organization.

Steps to reproduce:

• Navigate to the qtbase\tests\auto\tools\qmake\testdata\simple_dll project.
• Run "qmake -tp vc" to generate a VS project. Open project and verify that _WINDLL is defined when building.
• Run "qmake" to generate a makefile. Verify that _WINDLL is NOT defined when building with "jom".