[CMake] ExternalProject_Add and include_external_msproject dynamic duo... but what if their powers combined?

[Brian J. Davis]

Every time I create a superbuild using ExternalProject_Add sooner or 
later a project will not support cmake, but will have buried within its 
bowels  a visual studio solution project .sln and sometimes gobs of 
.vcxproj files (read CPython).  Now ofcourse include_external_msproject 
can do this.  If only these two wonder twins 
(https://en.wikipedia.org/wiki/Wonder_Twins) could work together in the 
same ExternalProject_Add command.  Maybe by adding after HG_TAG say 
VCX_PROJS or some such.

Only thing better would be getting original project maintainer to 
support CMake.

So feature request: Can we get ExternalProject_Add to support visual 
studio project files?