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.

2 files changed, 2 insertions, 2 deletions

diff --git a/src/prog.mk b/src/prog.mk
index b7ac59785..426961c29 100644
--- a/src/prog.mk
+++ b/src/prog.mk
@@ -13,7 +13,7 @@ OBJS := $(SRCS:.c=.o) $(EXTRA_OBJS)
 all: $(TARGET)
 
 %.o : %.c $(HDRS) $(ROOT)/config.mk
-        $(CC) $(PROG_CFLAGS) $(CFLAGS) $(INCLUDE) -c $< -o $@
+        $(CC) $(PROG_CFLAGS) $(CFLAGS) $(CPPFLAGS) -c $< -o $@
 
 $(PROG): $(OBJS) $(ROOT)/config.mk
         $(CC) $(PROG_LDFLAGS) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
diff --git a/src/so.mk b/src/so.mk
index c1913b794..27c5695a8 100644
--- a/src/so.mk
+++ b/src/so.mk
@@ -13,7 +13,7 @@ OBJS := $(SRCS:.c=.o) $(EXTRA_OBJS)
 all: $(TARGET)
 
 %.o : %.c $(HDRS) $(ROOT)/config.mk
-        $(CC) $(SO_CFLAGS) $(CFLAGS) $(INCLUDE) -c $< -o $@
+        $(CC) $(SO_CFLAGS) $(CFLAGS) $(CPPFLAGS) -c $< -o $@
 
 $(SO): $(OBJS) $(ROOT)/config.mk
         $(CC) $(SO_LDFLAGS) -shared $(LDFLAGS) -o $@ $(OBJS) -ldl