Set INTERFACE_COMPILE_DEFINITIONS of imported gflags library targets to define GFLAGS_DLL_DECL

[GoogleCodeExporter]

CMake's 
[http://www.cmake.org/cmake/help/v2.8.12/cmake.html#prop_tgt:INTERFACE_COMPILE_D
EFINITIONS INTERFACE_COMPILE_DEFINITIONS] property on the imported targets can 
be used to specify the proper definition of GFLAGS_DLL_DECL if both shared and 
static libraries were build. In this case, both libraries require the inclusion 
of the same public header files, but with different GFLAGS_DLL_DECL in case of 
the VS compiler.

Original issue reported on code.google.com by [email protected] on 25 Mar 2014 at 3:34