doc: spliting building steps for unix and macos

Currently, there is an error when executing export: CXX=g++-12 on macOS. Without it, it compiles fine. So, we split the step into two caterogies: steps for unix and steps for macOS

Fixes: #58219

Restoring previous BUILDING.md

new doc proposal for g++-12 export command

removing non related line change

restoring end of file

restoring comment proposal

Author: vagostep

BUILDING.md

@@ -246,6 +246,12 @@ Installation via Linux package manager can be achieved with:
 
 FreeBSD and OpenBSD users may also need to install `libexecinfo`.
 
+If you're getting a warning related to g++ older version, you can use
+
+```bash
+export CXX=g++-12
+```
+
 #### macOS prerequisites
 
 * Xcode Command Line Tools >= 13 for macOS
@@ -266,7 +272,6 @@ fail.
 To build Node.js:
 
 ```bash
-export CXX=g++-12
 ./configure
 make -j4
 ```