gdal: failed to compile in x86_64-w64-mingw32.static target

without this changes "./configure" can not find libjson-c at least in the x86_64-w64-mingw32.static target (at master branch)
I don't really like these last three hunks:

- The backtick format is widely used in the mxe codebase, and is supported by all major (all?) shells
- `$(shell)` is evaluated when the makefile is scanned, which means that it might cause unpredictable results, like not being able to find the package as when you run `make gdal` openssl is not installed.

Can you paste the log of failed build without these three hunks?