Doxygen broken with addition of fypp everywhere.

[sbryngelson]

Describe the bug
Doxygen doesn't recognize fypp files and thus doesn't build documentation for them.

To Reproduce
Steps to reproduce the behavior:

1. Navigate here

Expected behavior
Documentation should be produced for all source files.
If fypp and Doxygen do not play nicely, one can autogenerate the code and use Doxygen on those files.

[henryleberre]

The issue is here:

MFC/CMakeLists.txt

Line 348 in 95ac09d

"${${target}_srcs}"

I must have changed _srcs to be _SRCs one day and forgot to update it there. This would have still worked if the documentation action had built the codes because the fpp files would be declared as target dependencies and would have generated F90 files into src/${target}/autogen.