build: fix -Wunused-but-set-variable support and handle clang better
By default the build used -Wno-unused-but-set-variable but that's clearly not what we want. Fix those problems.
When using clang, the compiler doesn't error on unknown compile options, so let's add -Werror to the checking too.
CC='clang' configure:3408: checking for gcc configure:3435: result: clang ... configure:17644: checking whether gcc understands -Wno-unused-but-set-variable configure:17657: clang -c -Wall -std=gnu89 -g -O2 -Wmissing-declarations -Wmissing-prototypes -Wdeclaration-after-statement -Wstrict-prototypes -Wno-unused-parameter -Wno-sign-compare -Wno-deprecated-declarations -Wno-unused-but-set-variable conftest.c >&5 warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option] 1 warning generated. configure:17657: $? = 0 configure:17666: result: yes
Reported-by: Torsten Hilbrich torsten.hilbrich@secunet.com