make: Build with -Werror by default

Can be overridden from the environment by setting WERROR=0

also remove reversed logic for Werror in Makefile.buildtest
(a typo caused Werror to be always enabled for all builds)

Makefile.buildtests

@@ -31,11 +31,6 @@ endif
         info-buildsizes-diff info-build info-boards-supported \
         info-features-missing info-boards-features-missing
 
-# Make buildtests error out on warnings by adding -Werror
-ifeq (, $(filter buildtest, $(MAKECMDGOALS)))
-    CFLAGS+=-Werror
-endif
-
 ifeq ($(BUILD_IN_DOCKER),1)
 buildtest: ..in-docker-container
 else
@@ -63,6 +58,7 @@ buildtest:
 					BINDIRBASE=$${BINDIRBASE} \
 					RIOTNOLINK=$${RIOTNOLINK} \
 					RIOT_VERSION=$${RIOT_VERSION} \
+					WERROR=$${WERROR} \
 					$(MAKE) -j$(NPROC) 2>&1) ; \
 			if [ "$${?}" = "0" ]; then \
 				${COLOR_ECHO} "${COLOR_GREEN}success${COLOR_RESET}"; \

Makefile.docker

@@ -50,6 +50,7 @@ export DOCKER_ENV_VARS = \
   OFLAGS \
   PREFIX \
   QUIET \
+  WERROR \
   RIOT_VERSION \
   SCANBUILD_ARGS \
   SCANBUILD_OUTPUTDIR \

Makefile.include

@@ -64,10 +64,17 @@ else
 endif
 
 ifeq ($(QUIET),1)
-	AD=@
-	MAKEFLAGS += --no-print-directory
+  AD=@
+  MAKEFLAGS += --no-print-directory
 else
-	AD=
+  AD=
+endif
+
+# Fail on warnings. Can be overridden by `make WERROR=0`.
+WERROR ?= 1
+export WERROR
+ifeq ($(WERROR),1)
+  CFLAGS += -Werror
 endif
 
 ifneq (10,$(if ${RIOT_VERSION},1,0)$(if ${__RIOTBUILD_FLAG},1,0))

Makefile.scan-build

@@ -25,6 +25,7 @@ SCANBUILD_ENV_VARS := \
   PATH \
   PREFIX \
   QUIET \
+  WERROR \
   RIOT_VERSION \
   SIZE \
   TOOLCHAIN \

Makefile.vars

@@ -37,6 +37,7 @@ export OBJCOPY               # The command used to create the HEXFILE.
 export OFLAGS                # The parameter for OBJCOPY, e.g. to strip the debug information.
 export SIZE                  # The command to read to size of the ELF sections.
 export UNDEF                 # Set by the BOARD's and CPU's Makefile.include, this contains object files with must not be used in the ELFFILE even if the if no call to the functions.
+export WERROR                # Treat all compiler warnings as errors if set to 1 (see -Werror flag in GCC manual)
 
 export FLASHER               # The command to call on "make flash".
 export FFLAGS                # The parameters to supply to FLASHER.