Support static binary builds

[MaximeVdB]

By default, our binaries (executable and (shared) library) are dynamically linked to e.g. the libgfortran runtime library. This PR adds the option for static builds (with the STATIC=1 make option). So these are self-contained binaries which can then be released as precompiled standalone versions (at least the aquacrop executable; there is probably less interest in releasing a libaquacrop.a static library).

Performed tests:

• [x] Selected tests (perennial and harvest) with regular build (STATIC=0) are still passing with FORTRAN_EXE=0/1 and DEBUG=0/1 with both the Singularity image and the foss-2018a toolchain.
• [x] Compiling the executable with STATIC=1 on Genius and running the Perennial testcase with it works on a Ubuntu WSL2 on my local Windows laptop. The run time was about 3 seconds, which is similar to what I get on a Genius login node.
• [x] Compiling the executable with STATIC=1 and FC=x86_64-w64-mingw32-gfortran on a Ubuntu WSL2 on my local Windows laptop (MinGW v7.3) works. I can then execute the same Perennial test in a Windows Powershell, and the resulting outputs are identical to the reference. The run time was about 12 seconds, which is clearly slower than usual (but still acceptable?).

[gdelannoy]

The windows executable takes 12 sec and that seems very reasonable to me for the testcase. I need to say that I have no idea how much the official windows executable (only available one is v6, we can ask Dirk for v7) would take for the same task.