include: curl/system.h is a run-time version of curlbuild.h

system.h is aimed to replace curlbuild.h at a later point in time when
we feel confident system.h works sufficiently well.

curl/system.h is currently used in parallel with curl/curlbuild.h

curl/system.h determines a data sizes, data types and include file
status based on available preprocessor defines instead of getting
generated at build-time. This, in order to avoid relying on a build-time
generated file that makes it complicated to do 32 and 64 bit bields from
the same installed set of headers.

Test 1541 verifies that system.h comes to the same conclusion that
curlbuild.h offers.

Closes #1373