Before this patch CMake builds used two separate compilation passes to
build the shared and static libcurl respectively. This patch allows to
reduce that to a single pass if the target platform and build settings
allow it.
This reduces CMake build times when building both static and shared
libcurl at the same time, making these dual builds an almost zero-cost
option.
Enable this feature for Windows builds, where the difference between the
two passes was the use of `__declspec(dllexport)` attribute for exported
API functions for the shared builds. This patch replaces this method
with the use of `libcurl.def` at DLL link time.
Also update `Makefile.mk` to use `libcurl.def` to export libcurl API
symbols on Windows. This simplifies (or fixes) this build method (e.g.
in curl-for-win, which generated a `libcurl.def` from `.h` files using
an elaborate set of transformations).
`libcurl.def` has the maintenance cost of keeping the list of public
libcurl API symbols up-to-date. This list seldom changes, so the cost
is low.
Closes#11546
