cmake: define `WIN32_LEAN_AND_MEAN` for examples

cmake: define `WIN32_LEAN_AND_MEAN` for examples

To build faster. Also syncing with autotools.

Job                 | Before | After |
:------------------ | -----: | ----: |
dl-mingw 15         |    45s |   39s |
dl-mingw 9.5.0      |    72s |   49s |
dl-mingw 7.3.0      |    29s |   19s |
dl-mingw 6.4.0      |    20s |    9s |
Linux cross         |    30s |   19s |
MSVC UWP            |    77s |   65s |
MSVC x64            |    27s |   22s |
VS2010              |    61s |   48s |
VS2022 clang-cl     |   226s |  195s |

It also happens to fix what looks like a Windows SDK header bug seen
with VS2019 on AppVeyor CI:
```
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\winscard.h(1422,1): error C2220: the following warning is treated as an error (compiling source file C:\projects\curl\docs\examples\block_ip.c)
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\winscard.h(1422,1): warning C5031: #pragma warning(pop): likely mismatch, popping warning state pushed in different file (compiling source file C:\projects\curl\docs\examples\block_ip.c)
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\winioctl.h : message : #pragma warning(push) (compiling source file C:\projects\curl\docs\examples\block_ip.c)
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\windows.h(254,17): warning C5031: #pragma warning(pop): likely mismatch, popping warning state pushed in different file (compiling source file C:\projects\curl\docs\examples\block_ip.c)
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\winscard.h : message : #pragma warning(push) (compiling source file C:\projects\curl\docs\examples\block_ip.c)
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\winsock2.h(4221,1): warning C5031: #pragma warning(pop): likely mismatch, popping warning state pushed in different file (compiling source file C:\projects\curl\docs\examples\block_ip.c)
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\windows.h : message : #pragma warning(push) (compiling source file C:\projects\curl\docs\examples\block_ip.c)
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\um\winsock2.h : warning C5032: detected #pragma warning(push) with no corresponding #pragma warning(pop) (compiling source file C:\projects\curl\docs\examples\block_ip.c)
```
Ref: https://ci.appveyor.com/project/curlorg/curl/builds/52531106/job/66b6r098pll2le70#L312

Closes #18232