i stumbled across _COMPILING_NEWLIB and it seems to be what i want: a symbol
that indicates the code currently being compiled is newlib itself so that the
header can change behavior for that environment specifically.  is that what
it's meant for ?

if so, why does it seem to be inconsistently defined ?  newlib/configure.host
will add it for a few random targets, as does the mips-specific
newlib/libc/machine/mips/Makefile.am, as do a few specific winsup/cygwin/
files.  it feels like the patch below is what we should have.

if that's not what this is for, is there a define that has this meaning ?
in the glibc & gnulib world, the plain _LIBC define indicates this.
-mike

--- a/newlib/configure.host
+++ b/newlib/configure.host
@@ -54,7 +54,7 @@
 #   have_init_fini	have init/fini ("yes" or "no", set to "yes" by default)
 #   noinclude		list of include files to not install
 
-newlib_cflags=
+newlib_cflags="-D_COMPILING_NEWLIB"
 libm_machine_dir=
 machine_dir=
 shared_machine_dir=
@@ -467,15 +467,11 @@ case "${host}" in
 	sys_dir=a29khif
 	signal_dir=
 	;;
-  aarch64*-*-*)
-	newlib_cflags="${newlib_cflags} -D_COMPILING_NEWLIB"
-	;;
   amdgcn*)
 	sys_dir=amdgcn
 	have_crt0="no"
 	;;
   arm*-*-*)
-	newlib_cflags="${newlib_cflags} -D_COMPILING_NEWLIB"
 	sys_dir=arm
 	if [ "x${newlib_may_supply_syscalls}" = "xno" ] ; then
 	  have_crt0="no"
@@ -652,11 +648,11 @@ case "${host}" in
 	default_newlib_io_long_double="yes"
 	default_newlib_io_pos_args="yes"
 	CC="${CC} -I${cygwin_srcdir}/include"
-	newlib_cflags="${newlib_cflags} -DHAVE_OPENDIR -DHAVE_RENAME -DGETREENT_PROVIDED -DSIGNAL_PROVIDED -D_COMPILING_NEWLIB -DHAVE_BLKSIZE -DHAVE_FCNTL -DMALLOC_PROVIDED"
+	newlib_cflags="${newlib_cflags} -DHAVE_OPENDIR -DHAVE_RENAME -DGETREENT_PROVIDED -DSIGNAL_PROVIDED -DHAVE_BLKSIZE -DHAVE_FCNTL -DMALLOC_PROVIDED"
 	syscall_dir=syscalls
 	;;
   *-*-phoenix*)
-	newlib_cflags="${newlib_cflags} -DMISSING_SYSCALL_NAMES -D_COMPILING_NEWLIB -DHAVE_BLKSIZE -DHAVE_NANOSLEEP"
+	newlib_cflags="${newlib_cflags} -DMISSING_SYSCALL_NAMES -DHAVE_BLKSIZE -DHAVE_NANOSLEEP"
 	default_newlib_io_long_long="yes"
 	syscall_dir=
 	;;
@@ -671,7 +667,6 @@ case "${host}" in
 	default_newlib_io_long_long="yes"
 	default_newlib_io_c99_formats="yes"
 	newlib_cflags="${newlib_cflags} -ffunction-sections -fdata-sections "
-	newlib_cflags="${newlib_cflags} -D_COMPILING_NEWLIB"
 newlib_cflags="${newlib_cflags} -DCLOCK_PROVIDED -DMALLOC_PROVIDED -DEXIT_PROVIDED -DSIGNAL_PROVIDED -DGETREENT_PROVIDED -DREENTRANT_SYSCALLS_PROVIDED -DHAVE_NANOSLEEP -DHAVE_BLKSIZE -DHAVE_FCNTL -DHAVE_ASSERT_FUNC"
         # turn off unsupported items in posix directory 
 	newlib_cflags="${newlib_cflags} -D_NO_GETLOGIN -D_NO_GETPWENT -D_NO_GETUT -D_NO_GETPASS -D_NO_SIGSET -D_NO_WORDEXP -D_NO_POPEN -D_NO_POSIX_SPAWN"
--- a/newlib/libc/machine/mips/Makefile.am
+++ b/newlib/libc/machine/mips/Makefile.am
@@ -9,8 +9,6 @@ AM_CCASFLAGS = $(INCLUDES)
 noinst_LIBRARIES = lib.a
 
 lib_a_SOURCES = setjmp.S strlen.c strcmp.S strncpy.c memset.S memcpy.S
-lib_a_CCASFLAGS=$(AM_CCASFLAGS) -D_COMPILING_NEWLIB
-lib_a_CFLAGS=$(AM_CFLAGS) -D_COMPILING_NEWLIB
 
 ACLOCAL_AMFLAGS = -I ../../.. -I ../../../..
 CONFIG_STATUS_DEPENDENCIES = $(newlib_basedir)/configure.host
--- a/newlib/libc/machine/mips/Makefile.in
+++ b/newlib/libc/machine/mips/Makefile.in
@@ -198,8 +198,6 @@ INCLUDES = $(NEWLIB_CFLAGS) $(CROSS_CFLAGS) $(TARGET_CFLAGS)
 AM_CCASFLAGS = $(INCLUDES)
 noinst_LIBRARIES = lib.a
 lib_a_SOURCES = setjmp.S strlen.c strcmp.S strncpy.c memset.S memcpy.S
-lib_a_CCASFLAGS = $(AM_CCASFLAGS) -D_COMPILING_NEWLIB
-lib_a_CFLAGS = $(AM_CFLAGS) -D_COMPILING_NEWLIB
 ACLOCAL_AMFLAGS = -I ../../.. -I ../../../..
 CONFIG_STATUS_DEPENDENCIES = $(newlib_basedir)/configure.host
 all: all-am