This patch updates m4/compiler-warnings.m4 as follows to make it more
compatible with clang:
* clang doesn't support -Wunused-but-set-variable, which can be replaced
with -Wunused-variable as the latter covers unused variables with or
without an assigned value and is supported by gcc and clang.
* -Wno-unused-function is added to let clang ignore unused functions that
are auto-generated.
9dac5ece77