cmake: add BORINGSSL_VERSION configuration variable, test in CI

To simplify setting BoringSSL version, using:
`-DBORINGSSL_VERSION=0.20260211.0`
or
`-DBORINGSSL_VERSION=${boringssl_version}`

Previously it could be set via C flags, using complicated shell quotes:
`-DCMAKE_C_FLAGS="-DCURL_BORINGSSL_VERSION=\\\"${boringssl_version}\\\""`
(the C flags method remains, also for autotools)

It'd be nice if BoringSSL published its version not just via
`MODULE.bazel` in its source tree, but from its public headers, to make
these workarounds unnecessary.

Also:
- GHA/http3-linux: test both options.

Closes #20571

.github/workflows/http3-linux.yml

@@ -639,14 +639,16 @@ jobs:
         run: |
           [ -n "${MATRIX_PKG_CONFIG_PATH}" ] && export PKG_CONFIG_PATH="${MATRIX_PKG_CONFIG_PATH}"
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
+            [[ "${MATRIX_GENERATE}" = *'boringssl'* ]] && options=" -DBORINGSSL_VERSION=${BORINGSSL_VERSION}"
             cmake -B bld -G Ninja \
               -DCMAKE_C_COMPILER_TARGET="$(uname -m)-pc-linux-gnu" -DBUILD_STATIC_LIBS=ON \
               -DCURL_WERROR=ON -DENABLE_DEBUG=ON \
               -DCURL_USE_LIBUV=ON \
               -DTEST_NGHTTPX=/home/runner/nghttp2/build/bin/nghttpx \
               -DHTTPD_NGHTTPX=/home/runner/nghttp2/build/bin/nghttpx \
-              ${MATRIX_GENERATE}
+              ${MATRIX_GENERATE} ${options}
           else
+            [[ "${MATRIX_CONFIGURE}" = *'boringssl'* ]] && export CPPFLAGS="-DCURL_BORINGSSL_VERSION=\\\"${BORINGSSL_VERSION}\\\""
             mkdir bld && cd bld && ../configure --enable-warnings --enable-werror --enable-debug --disable-static \
               --disable-dependency-tracking --enable-option-checking=fatal \
               --with-libuv \