[PATCH] Allow CPPFLAGS to be set on the make command line

[Tom Tromey <tom@tromey.com>]

While looking into PR build/8751 (which seems to be fixed), I noticed
that it's not possible to change CPPFLAGS for gdb on the "make"
command line.  It's reasonable to want to do this sometimes, and I
think this patch should suffice.

gdb/ChangeLog
2018-08-07  Tom Tromey  <tom@tromey.com>

	* Makefile.in (CPPFLAGS): New variable.
	(INTERNAL_CPPFLAGS): Use it.
---
 gdb/ChangeLog   | 5 +++++
 gdb/Makefile.in | 3 ++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 8c744d70c0..7ff817d790 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -557,6 +557,7 @@ PROFILE_CFLAGS = @PROFILE_CFLAGS@
 # when running make.  I.E.: "make CFLAGS=-Wmissing-prototypes".
 CFLAGS = @CFLAGS@
 CXXFLAGS = @CXXFLAGS@
+CPPFLAGS = @CPPFLAGS@
 
 # Set by configure, for e.g. expat.  Python installations are such that
 # C headers are included using their basename (for example, we #include
@@ -564,7 +565,7 @@ CXXFLAGS = @CXXFLAGS@
 # are sometimes a little generic, we think that the risk of collision
 # with other header files is high.  If that happens, we try to mitigate
 # a bit the consequences by putting the Python includes last in the list.
-INTERNAL_CPPFLAGS = @CPPFLAGS@ @GUILE_CPPFLAGS@ @PYTHON_CPPFLAGS@
+INTERNAL_CPPFLAGS = $(CPPFLAGS) @GUILE_CPPFLAGS@ @PYTHON_CPPFLAGS@
 
 # INTERNAL_CFLAGS is the aggregate of all other *CFLAGS macros.
 INTERNAL_CFLAGS_BASE = \
-- 
2.13.6