Make DFTD4 & multicharge dependencies optional

In this recent commit DFTD4 and multicharge were added as dependencies for xTB. As far as I can tell, these dependencies aren't currently used (all unittests pass without them, as well as a variety of individual calculations that I have tried).

If the intention is to eventually have DFTD4 and multicharge be required dependencies, this isn't toggling isn't strictly needed, but is nice to have in the interim until features that use that use these dependencies are added.

My motivation for making the pull request was that I hit a build failure on Windows (ifort/msvc compilers) trying to compile the multicharge dependency:

[stdout] [238/690] Compiling Fortran object subprojects/multicharge/test/tester.exe.p/test_wignerseitz.f90.obj
[stdout] [239/690] Compiling Fortran object subprojects/dftd4/dftd4-3.dll.p/src_dftd4_data.f90.obj
[stdout] [240/690] Compiling Fortran object subprojects/dftd4/dftd4-3.dll.p/src_dftd4_utils.f90.obj
[stdout] [241/690] Compiling Fortran object subprojects/multicharge/app/multicharge.exe.p/main.f90.obj
[stdout] [242/690] Linking target subprojects/multicharge/app/multicharge.exe
[stdout] FAILED: subprojects/multicharge/app/multicharge.exe
[stdout] "xilink.exe"  /MACHINE:x64 /OUT:subprojects/multicharge/app/multicharge.exe subprojects/multicharge/app/multicharge.exe.p/main.f90.obj "/nologo" "-STACK:16777216" "subprojects/multicharge/multicharge.lib" "subprojects/mctc-lib/libmctc-lib.a" "subprojects/json-fortran-8.2.5/libjsonfortran.a" "mkl_rt.lib" "/SUBSYSTEM:CONSOLE"
[stdout] LINK : fatal error LNK1181: cannot open input file 'subprojects\multicharge\multicharge.lib'

I assume this is a similar issue to https://github.com/grimme-lab/xtb/issues/646, where some symbols need to be exported in a Windows specific way. But I didn't want to dig too deep into the issue until I was sure that multicharge was actually being used by xTB.

grimme-lab / xtb

Make DFTD4 & multicharge dependencies optional #1081