cmake: add support for single libcurl compilation pass

[vszakats]

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

---

• curl previously had a libcurl.def placed in the lib directory. I opted for the root directory, because in lib it may be overwritten by .def files generated by the DLL linker command when using certain build methods/options.

• single-pass libcurl builds can be freely extended to other platforms, if both shared/static libs have position-independent code enable and symbol exports can be fixed up via a linker option.

• it might be an option to make libcurl.def an OS-agnostic raw list of public API symbols (= without the top EXPORTS line) for re-use for other purposes. CMake could handle that with some extra lines. In Makefile.mk less trivially, but also seems doable.

[dfandrich]

I thought the whole libcurl.def thing was unneeded because we're using __declspec(dllexport) on all the relevant exports.

[dfandrich]

It was removed the last time in commit 9bd7f00

[vszakats]

We do use declspec(), yes, and this is what forces separate compile passes for shared and static libs. The goal of this PR is to avoid that double work. Another positive side effect is that Makefile.mk builds can also use this libcurl.def, without having to generate it manually. Makefile.mk always used a single compile pass, so a .def file was always necessary for correct results (on Windows).

The file was deleted because it wasn't used. This time it will be used by two build systems.