cmake: support building static and shared libcurl in one go

This patch adds the ability to build a static and shared libcurl library
in a single build session. It also adds an option to select which one to
use when building the curl executable.

New build options:
- `BUILD_STATIC_LIBS`. Default: `OFF`.
  Enabled automatically if `BUILD_SHARED_LIBS` is `OFF`.
- `BUILD_STATIC_CURL`. Default: `OFF`.
  Requires `BUILD_STATIC_LIBS` enabled.
  Enabled automatically if building static libcurl only.
- `STATIC_LIB_SUFFIX`. Default: empty.
- `IMPORT_LIB_SUFFIX`. Default: `_imp` if implib filename would collide
  with static lib name (typically with MSVC) in Windows builds.
  Otherwise empty.

Also:

- Stop setting the `CURL_STATICLIB` macro via `curl_config.h`, and pass
  it directly to the compiler. This also allows to delete a condition
  from `tests/server/CMakeLists.txt`.

- Complete a TODO by following the logic used in autotools (also for
  `LIBCURL_NO_SHARED`), and set `-DCURL_STATICLIB` in `Cflags:` of
  `libcurl.pc` for _static-only_ curl builds.

- Convert an existing CI test to build both shared and static libcurl.

Closes #11505

.github/workflows/macos.yml

@@ -180,7 +180,7 @@ jobs:
           generate: -DOPENSSL_ROOT_DIR=/usr/local/opt/libressl -DCURL_DISABLE_LDAP=ON -DCURL_DISABLE_LDAPS=ON -DCMAKE_UNITY_BUILD=ON
         - name: libssh2
           install: nghttp2 openssl libssh2
-          generate: -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl -DCURL_USE_LIBSSH2=ON
+          generate: -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl -DCURL_USE_LIBSSH2=ON -DBUILD_SHARED_LIBS=ON -DBUILD_STATIC_LIBS=ON
     steps:
     - run: echo libtool autoconf automake pkg-config ${{ matrix.build.install }} | xargs -Ix -n1 echo brew '"x"' > /tmp/Brewfile
       name: 'brew bundle'