kiwix-desktop needs the aria2 binary to download content.
kiwix-desktop will use aria2 throw lib-kiwix so it is somehow a kiwix-lib
dependency, but it is used only by kiwix-desktop.
And we know to install it only on platforms where we are supporting
kiwix-destkop.
On ubuntu, the libcurl package is compiled with libkrb (kerberos).
However, no libkrb5.a is provided and so, we cannot link statically.
As we are not using kerberos, we can (and must) compile ourselves libcurl.
The CI is using ubuntu artful and the deb package is compiled with another
compiler that gcc-4.8 (used in other project CI).
As we compile everything with gcc-4.8, we must compile our own ctpp2.
Introduce also a "NeutralEnv", a build environment independent of the
targeted platform. All `Source` now build using the neutralEnv.
Most of toolchains are also using neutralEnv except android_ndk who is
specific to a platform.
As toolchain are neutral, platform specific environment variables are now
set by the platformInfo directly instead of the toolchain.
Matthieu Gautier