build: use CPPFLAGS instead of INCLUDE in compile targets

With this, CFLAGS and CPPFLAGS are used when compiling and LDFLAGS when
linking, just like in the built-in GNU make rules.  From `make -p`:

    COMPILE.c = $(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c
    LINK.c = $(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $(TARGET_ARCH)
    LINK.o = $(CC) $(LDFLAGS) $(TARGET_ARCH)

Note: It is unclear where the `INCLUDE` variable comes from; it is not
documented in autoconf nor GNU make and automake (which itself is not
used in this repository) only mentions `INCLUDES`:

    `INCLUDES`
         This does the same job as `AM_CPPFLAGS` (or any per-target
         `_CPPFLAGS` variable if it is used).  It is an older name for
         the same functionality.  This variable is deprecated; we
         suggest using `AM_CPPFLAGS` and per-target `_CPPFLAGS` instead.

Environment: automake 1.16.5-2 and GNU make 4.4.1 on Artix Linux.

See also commit 671c3f249 ("build: actually set LDFLAGS and LIBS in
makefiles", 2022-11-30) / PR #5504.

1 files changed, 1 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index fc99820de..d3b05d839 100644
--- a/configure.ac
+++ b/configure.ac
@@ -276,6 +276,7 @@ cat <<EOF
 Compile options:
    CC: $CC
    CFLAGS: $CFLAGS
+   CPPFLAGS: $CPPFLAGS
    LDFLAGS: $LDFLAGS
    EXTRA_CFLAGS: $EXTRA_CFLAGS
    EXTRA_LDFLAGS: $EXTRA_LDFLAGS