It seems that last meson version (0.40.0) do not pass cross compilation
extra_args to compiler when checking for headers.
Thus, the kiwix-lib check for ctpp2 headers fails because the WIN32
define is not present. So kiwix-lib is compiled without CTPP2 but
kiwix-tool compile as if kiwix-lib where compile with CTPP2 and fails.
By making CTPP2 headers check for `__WINGW32__` we workaround this problem.
I don't know if this is a bug in meson, ctpp2 or kiwix-lib but this small
fix allow to compile correctly with last meson version.
