General: Fix recent menuselect-related cross compile regression

MAKE_MENUSELECT currently sets CC to CC, which is the compiler for the
target platform. But menuselect is to be run on the build system, so
BUILD_CC needs to be used instead - like it was in the past, before the
recent changes (https://reviewboard.asterisk.org/r/4370/). This is the
patch for ASTERISK-25074.

ASTERISK-25074 #close
Reported by: Sebastian Kemper
Tested by: Sebastian Kemper

Change-Id: I8a2b1fc5deb6ad2b80f49baca35b1b13d468ebf8