6648 illumos build should be explicit about C standards

*** 21,37 ****
  
  ASSUFFIX_sparc  = S
  ASSUFFIX_i386   = s
  ASSUFFIX        = $(ASSUFFIX_$(MACH))
  
! # C99MODE of neither enabled nor disabled is "no_lib", whereby we expect
! # C99-the-language, but don't modify the behaviour of library routines.  This
! # is VERY IMPORTANT, as -xc99=%all, for instance, would link us with
  # values-xpg6, which would introduce an __xpg6 to our object with the C99
  # flags set, causing us to default C99 libm behaviour on, breaking
  # compatibility.
  C99MODE         =
  
  M4FLAGS         = -D__STDC__ -DPIC
  
  LDBLDIR_sparc   = Q
  LDBLDIR_i386    = LD
--- 21,43 ----
  
  ASSUFFIX_sparc  = S
  ASSUFFIX_i386   = s
  ASSUFFIX        = $(ASSUFFIX_$(MACH))
  
! # With studio C99MODE of neither enabled nor disabled is "no_lib", whereby we
! # expect C99-the-language, but don't modify the behaviour of library routines.
! # This is VERY IMPORTANT, as -xc99=%all, for instance, would link us with
  # values-xpg6, which would introduce an __xpg6 to our object with the C99
  # flags set, causing us to default C99 libm behaviour on, breaking
  # compatibility.
+ #
+ # We must then, unfortunately, defeat the GNU compiler _defaulting_ to C99, by
+ # in that case setting it back to gnu89, which _also_ accepts C99 syntax as
+ # far as is important.
  C99MODE         =
+ CFLAGS          += -_gcc=-std=gnu89
+ CFLAGS64        += -_gcc=-std=gnu89
  
  M4FLAGS         = -D__STDC__ -DPIC
  
  LDBLDIR_sparc   = Q
  LDBLDIR_i386    = LD
*** 47,57 ****
                  -I$(LIBMSRC)/$(LDBLDIR) -I$(LIBMDIR)/$(TARGET_ARCH)/src
  
  # GCC needs __C99FEATURES__ such that the implementations of isunordered,
  # isgreaterequal, islessequal, etc, exist.  This is basically equivalent to
  # providing no -xc99 to Studio, in that it gets us the C99 language features,
! # but not values-xpg6, the reason for which is outline with C99MODE.
  CFLAGS          += -_gcc=-D__C99FEATURES__
  CFLAGS64        += -_gcc=-D__C99FEATURES__
  
  # libm depends on integer overflow characteristics
  CFLAGS          += -_gcc=-fno-strict-overflow
--- 53,63 ----
                  -I$(LIBMSRC)/$(LDBLDIR) -I$(LIBMDIR)/$(TARGET_ARCH)/src
  
  # GCC needs __C99FEATURES__ such that the implementations of isunordered,
  # isgreaterequal, islessequal, etc, exist.  This is basically equivalent to
  # providing no -xc99 to Studio, in that it gets us the C99 language features,
! # but not values-xpg6, the reason for which is outlined with C99MODE.
  CFLAGS          += -_gcc=-D__C99FEATURES__
  CFLAGS64        += -_gcc=-D__C99FEATURES__
  
  # libm depends on integer overflow characteristics
  CFLAGS          += -_gcc=-fno-strict-overflow