Bump our language requirements to gnu11 & gnu++17

.. this is supported in buster (gcc8) and newer

src/Makefile

@@ -245,17 +245,15 @@ INCLUDE += $(LIBTIRPC_CFLAGS)
 INTEGER_OVERFLOW_FLAGS := -fwrapv
 OPT := -Os $(INTEGER_OVERFLOW_FLAGS)
 DEBUG := $(DEBUG) -g -Wall -Wno-stringop-truncation
-CFLAGS := $(INCLUDE) $(OPT) $(DEBUG) $(EXTRA_DEBUG) -DULAPI -std=gnu99 -fgnu89-inline -Werror=implicit-function-declaration $(CFLAGS) $(CPPFLAGS)
-CXXFLAGS := $(INCLUDE) $(EXTRA_DEBUG) -DULAPI $(DEBUG) $(OPT) -Woverloaded-virtual $(CXXFLAGS) $(CPPFLAGS)
-CXXFLAGS += $(call cxx-option, -Wno-psabi)
-CXXFLAGS += $(call cxx-option, -std=gnu++11, -std=gnu++0x)
+CFLAGS := $(INCLUDE) $(OPT) $(DEBUG) $(EXTRA_DEBUG) -DULAPI -std=gnu11 -Werror=implicit-function-declaration $(CFLAGS) $(CPPFLAGS)
+CXXFLAGS := $(INCLUDE) $(EXTRA_DEBUG) -DULAPI $(DEBUG) $(OPT) -Werror=overloaded-virtual $(CXXFLAGS) $(CPPFLAGS)
+CXXFLAGS += -std=gnu++17
 # In Debian 11, any inclusion of <boost/python.hpp> leads to several
 # diagnostics from included headers about deprecated features. LinuxCNC does
 # not directly use these deprecated features, but it does use boost::python.
 # Silence the warnings just when they occur in files using boost/python.hpp by
 # adding SILENCE_BOOST_INTERNAL_DIAGNOSTICS_FLAGS to the object's EXTRAFLAGS
 SILENCE_BOOST_INTERNAL_DIAGNOSTICS_FLAGS = -DBOOST_ALLOW_DEPRECATED_HEADERS=1 -DBOOST_BIND_GLOBAL_PLACEHOLDERS=1
-
 CFLAGS   += $(TOOL_NML_FLAG)
 CXXFLAGS += $(TOOL_NML_FLAG)
 
@@ -877,7 +875,7 @@ EXTRA_CFLAGS := $(filter-out -ffast-math,$(RTFLAGS)) -D__MODULE__ -I$(BASEPWD)/.
 	-I$(BASEPWD)/emc/nml_intf -I$(BASEPWD)/emc/kinematics -I$(BASEPWD)/emc/tp -I$(BASEPWD)/emc/motion \
 	-DSEQUENTIAL_SUPPORT -DHAL_SUPPORT -DDYNAMIC_PLCSIZE -DRT_SUPPORT -DOLD_TIMERS_MONOS_SUPPORT -DMODBUS_IO_MASTER \
 	-fno-fast-math $(call cc-option,-mieee-fp) -fno-unsafe-math-optimizations \
-	-Wframe-larger-than=2560 -Wno-declaration-after-statement \
+	-Werror=frame-larger-than=2560 -Wno-declaration-after-statement \
 	$(INTEGER_OVERFLOW_FLAGS)
 ifneq ($(KERNELRELEASE),)
 ifeq ($(RTARCH):$(RTAI):$(filter $(RTFLAGS),-msse),x86_64:3:)

src/configure.ac

@@ -33,13 +33,15 @@ fi
 AC_PROG_CXX
 AC_PROG_INSTALL
 
-m4_include([m4/ax_cxx_compile_stdcxx_11.m4])
+m4_include([m4/ax_require_defined.m4])
+m4_include([m4/ax_cxx_compile_stdcxx.m4])
+m4_include([m4/ax_cxx_compile_stdcxx_17.m4])
 m4_include([m4/ax_python.m4])
 m4_include([m4/ax_python_devel.m4])
 m4_include([m4/ax_boost_base.m4])
 m4_include([m4/ax_boost_python.m4])
 # make this optional - it will likely only result in warnings
-AX_CXX_COMPILE_STDCXX_11(noext,optional)
+AX_CXX_COMPILE_STDCXX_17(ext,mandatory)
 
 AC_MSG_CHECKING(build toplevel)
 BUILD_TOPLEVEL="$(cd ..; pwd -P)"