Add option to force finding libftdi1 using pkg-config until MSYS2 pac…

[cr1901]

…kage is patched.

The LibFTDI1Config.cmake module in MSYS2/MINGW-packages hardcodes paths in a way that Ninja (at least) cannot find the dependency libusb when trying to build OFL. The old pkg-config method of finding the dep still works fine, however. So add an option to force the old way until a fix finds its way upstream.

(I'm taking a peek at it, but seems more difficult to fix than I thought.)

[cr1901]

Hmmm, maybe this isn't the proper fix. When configuring with pkg-config, I get:

CMake Error at CMakeLists.txt:832 (math):
  math cannot parse the expression: " * 100 + ": syntax error, unexpected
  exp_TIMES (2).

But probably better to fix it for pkg-config than to gate it out.

[trabucayre]

Thanks to point this issue.
In fact I have removed find_package: this approach is in many cases error prone.
See c6ca4e5
Could you test with an up to date git to see if this fix your issue?

[cr1901]

@trabucayre Your commit works fine. Closing this PR. Maybe in the future, find_package can be reintroduced... it seems more integrated w/ cmake, so in principle plays nicer with Windows (which aside from MSYS2 doesn't exactly have well-defined library locations). But that can be handled another day.