cmake, `Makefile.mk`: use `-isystem` for dep headers, silence BearSSL issues

cmake, `Makefile.mk`: use `-isystem` for dep headers, silence BearSSL issues

Patch started out for working around compiler warnings in BearSSL latest
tarball release v0.6 (2018-08-14) and Apple clang 14 with CMake.

Then turned into patching CMake and `Makefile.mk` builds to use
`-isystem` instead `-I` when adding header directories for
dependencies. This avoids compiler warnings in dependency headers,
syncing behaviour with autotools.

Also:
- `Makefile.mk`: add support for BearSSL.
- delete warning suppression for mbedTLS headers. No longer necessary
  after this patch.
  Follow-up to 434db995a7566a76f6a16391897816ddd1011adc #12720

Silenced BearSSL warnings:
```
In file included from curl/lib/vtls/bearssl.c:28:
In file included from bearssl/inc/bearssl.h:127:
bearssl/inc/bearssl_hash.h:727:5: warning: 'BR_DOXYGEN_IGNORE' is not defined, evaluates to 0 [-Wundef]
    ^
bearssl/inc/bearssl_hash.h:745:5: warning: 'BR_DOXYGEN_IGNORE' is not defined, evaluates to 0 [-Wundef]
    ^
In file included from curl/lib/vtls/bearssl.c:28:
In file included from bearssl/inc/bearssl.h:136:
bearssl/inc/bearssl_ssl.h:1253:20: warning: implicit conversion loses integer precision: 'unsigned int' to 'uint16_t' (aka 'unsigned short') [-Wimplicit-int-conversion]
        cc->version_min = version_min;
                        ~ ^~~~~~~~~~~
bearssl/inc/bearssl_ssl.h:1254:20: warning: implicit conversion loses integer precision: 'unsigned int' to 'uint16_t' (aka 'unsigned short') [-Wimplicit-int-conversion]
        cc->version_max = version_max;
                        ~ ^~~~~~~~~~~
bearssl/inc/bearssl_ssl.h:1327:28: warning: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'uint16_t' (aka 'unsigned short') [-Wimplicit-int-conversion]
        ctx->protocol_names_num = num;
                                ~ ^~~
5 warnings generated.
```

(These warnings were fixed in BearSSL Git master in 2019 via
2893441f2efd4603ddd6d7f49011bdda096a4a87 and
ecdf89770ee82dfea6186fb4369cff3d06cd852e.)

Also these two cases, which are caused by an unidentified component
(outside curl) cranking up MSVC warnings in external headers to `/W4`
when ZLIB is deselected:
https://github.com/curl/curl/pull/14859#issuecomment-2351809153

mbedTLS 3.6.1:
```
C:\vcpkg\installed\x64-windows\include\psa\crypto_struct.h(254,13): error C2220: the following warning is treated as an error [D:\a\curl\curl\bld\lib\libcurl_object.vcxproj]
  (compiling source file 'CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c')
C:\vcpkg\installed\x64-windows\include\psa\crypto_struct.h(254,13): warning C4200: nonstandard extension used: zero-sized array in struct/union [D:\a\curl\curl\bld\lib\libcurl_object.vcxproj]
  (compiling source file 'CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c')
```
Ref: https://github.com/curl/curl/actions/runs/10842694205/job/30107466989?pr=14859#step:10:29

nghttp3 1.5.0:
```
C:\vcpkg\installed\x64-windows\include\nghttp3\nghttp3.h(2678,1): error C2220: the following warning is treated as an error [D:\a\curl\curl\bld\lib\libcurl_object.vcxproj]
  (compiling source file 'CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c')
  C:\vcpkg\installed\x64-windows\include\nghttp3\nghttp3.h(2678,1): warning C4324: 'nghttp3_pri': structure was padded due to alignment specifier [D:\a\curl\curl\bld\lib\libcurl_object.vcxproj]
  (compiling source file 'CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c')
```
Ref: https://github.com/curl/curl/actions/runs/10871875297/job/30166233862?pr=14859#step:10:28

Closes #14763