rtai_math.ko and musl: Switch from gnu89/C99 to GNU11

The RTAI math kernel module (rtai_math.ko) was originally
built with gnu89, with musl itself being built with C99.
This is because musl is built outside of Kbuild
(therefore not a kernel module) and then is linked to
rtai_math.ko as the final step.

musl's libm libary provides the nitty gritty of math functions
which are used and exported by rtai_math.ko so it can be used
by external projects such as LinuxCNC.

This commit forces both the RTAI math module (rtai_math.ko)
and musl itself to pure GNU11.

With this, no modification to LinuxCNC is required. LinuxCNC
uses `rtai-config --module-cflags` to capture the appropriate
compiler flags to pass to it's Kbuild system (what builds the
LinuxCNC kernel modules.) The `--module-cflags` option
represents RTAI_FP_CFLAGS.

Signed-off-by: Alec Ari <neotheuser@ymail.com>

configure.ac

@@ -741,7 +741,7 @@ RTAI_KBUILD_DISTCLEAN=""
 
 dnl Extra options to be passed to the compiler
 RTAI_USER_CFLAGS="-O2 -march=x86-64 -fomit-frame-pointer -pipe"
-RTAI_FP_CFLAGS="${RTAI_USER_CFLAGS} -D__IN_RTAI__ -mcmodel=kernel -nostdinc -ffreestanding -fno-pic -fno-builtin -fno-common -fno-unwind-tables -fno-asynchronous-unwind-tables -frounding-math -mpreferred-stack-boundary=4 -mstackrealign -mfpmath=sse -msse -msse2"
+RTAI_FP_CFLAGS="${RTAI_USER_CFLAGS} -D__IN_RTAI__ -mcmodel=kernel -std=gnu11 -nostdinc -ffreestanding -fno-pic -fno-builtin -fno-common -fno-unwind-tables -fno-asynchronous-unwind-tables -frounding-math -mpreferred-stack-boundary=4 -mstackrealign -mfpmath=sse -msse -msse2"
 
 AC_SUBST(RTAI_TARGET_SUBARCH)
 AC_SUBST(RTAI_BUILTIN_MODLIST)

src/math/Makefile.kbuild

@@ -15,7 +15,7 @@ EXTRA_CFLAGS += -I$(rtai_srctree)/include \
 		$(rtai_extradef) \
 		-D__IN_RTAI__ \
 		-D_IEEE_LIBM \
-		-D_ISOC99_SOURCE \
+		-D_ISOC11_SOURCE \
 		-D_DEFAULT_SOURCE
 
 obj-m += rtai_math.o

src/math/musl/GNUmakefile.am

@@ -9,7 +9,7 @@ BASE_OBJS = $(patsubst $(srcdir)/%,%.o,$(basename $(BASE_SRCS)))
 ALL_OBJS = $(addprefix obj/, $(sort $(BASE_OBJS)))
 LIBM_OBJS = $(filter obj/%,$(ALL_OBJS))
 
-CFLAGS = @RTAI_FP_CFLAGS@ -std=c99 -pedantic
+CFLAGS = @RTAI_FP_CFLAGS@ -pedantic
 CFLAGS += -ffunction-sections -fdata-sections -Iobj/include -I$(srcdir)/include
 CFLAGS += -D_XOPEN_SOURCE=700 -Wa,--noexecstack -Wall -Werror
 CFLAGS += -Wno-parentheses -Wno-unused-but-set-variable -Wno-maybe-uninitialized