cobol: Remove unnecesssary CPPFLAGS update and restore MacOS build

The build currently fails on MacOS even when the Cobol front-end and
libgcobol builds are disabled.

The problem is that gcc/cobol/Make-lang.in adds -Iinclude to CPPFLAGS,
which somehow makes clang unhappy about the include order:
  error: <cstddef> tried including <stddef.h> but didn't find libc++'s
  <stddef.h> header. This usually means that your header search paths
  are not configured properly.

It turns out that this addition is unnecessary: simply removing it fixes
the build on MacOS, without impacting the build x86_64-pc-linux-gnu when
configured with --enable-languages=default,cobol.

It feels like there might be more cleanup opportunities there, but they
can be taken care of later.

gcc/cobol/ChangeLog:

	* Make-lang.in: Remove unnecessary CPPFLAGS update.

gcc/cobol/Make-lang.in

@@ -56,7 +56,6 @@ LIB_SOURCE ?= $(srcdir)/../libgcobol
 #
 CPPFLAGS =					\
  -std=c++14					\
- -Iinclude					\
  -I$(BINCLUDE)					\
  -I$(LIB_INCLUDE)				\
  -DEXEC_LIB=\"$(prefix)/lib64\"		\