message: doc: remove CC_FOR_BUILD from OpenBSD build doc
  This is no-longer necessary after upstream libsecp256k1 changes.
diff: @@ -25,8 +25,8 @@ See [dependencies.md](dependencies.md) for a complete overview.
  **Important**: From OpenBSD 6.2 onwards a C++11-supporting clang compiler is
  part of the base image, and while building it is necessary to make sure that
  this compiler is used and not ancient g++ 4.2.1. This is done by appending
  -`CC=cc CC_FOR_BUILD=cc CXX=c++` to configuration commands. Mixing different
  -compilers within the same executable will result in errors.
  +`CC=cc CXX=c++` to configuration commands. Mixing different compilers within
  +the same executable will result in errors.
  ### Building BerkeleyDB
  @@ -84,7 +84,7 @@ To configure with wallet:
  To configure without wallet:
  ```bash
  -./configure --disable-wallet --with-gui=no --disable-external-signer CC=cc CC_FOR_BUILD=cc CXX=c++ MAKE=gmake
  +./configure --disable-wallet --with-gui=no --disable-external-signer CC=cc CXX=c++ MAKE=gmake
  ```
  To configure with GUI:
  