gecko: unbreak with non-default CC/CXX

checking for clang for bindgen... not found
checking for libclang for bindgen... not found
ERROR: Could not find clang to generate run bindings for C/C++. Please install the necessary packages, run mach bootstrap, or use --with-clang-path to give the location of clang.

checking for llvm-objdump... not found
DEBUG: llvm_objdump: Trying llvm-objdump
ERROR: Cannot find llvm-objdump

Reported by: pkg-fallout (powerpc64)