Merge bitcoin/bitcoin#30785: doc: fix compiler flags for macOS configuration

8d7f8fa doc: fix compiler flags for macOS configuration (marcofleon)

Pull request description:

  Small CMake correction in the macOS build docs. My Mac will actually use AppleClang if I don't specify the compiler flags.
  ```bash
  % cmake -B testbuild
  -- The CXX compiler identification is AppleClang 15.0.0.15000309
  ```

  ```bash
  % cmake -B testbuild -DCMAKE_C_COMPILER="$(brew --prefix llvm)/bin/clang" -DCMAKE_CXX_COMPILER="$(brew --prefix llvm)/bin/clang++"
  -- The CXX compiler identification is Clang 18.1.8
  ```

ACKs for top commit:
  maflcko:
    review ACK 8d7f8fa
  brunoerg:
    ACK 8d7f8fa

Tree-SHA512: f25f0b38d5868d3182b79b8d0ff75ba7041e6282a6f4c83ed08ba7254ec9d8ae2553df36a4654a91ede5777e4c727dbdce6dcee7a5fe35bdd403d17e249ddd18

Author: fanquake

doc/build-osx.md

@@ -60,7 +60,7 @@ brew install llvm
 And append the following to the configure commands below:
 
 ``` bash
-CC=$(brew --prefix llvm)/bin/clang CXX=$(brew --prefix llvm)/bin/clang++
+-DCMAKE_C_COMPILER="$(brew --prefix llvm)/bin/clang" -DCMAKE_CXX_COMPILER="$(brew --prefix llvm)/bin/clang++"
 ```
 
 Try `llvm@17` if compilation fails with the default version of llvm.