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 671c3f2 ("build: actually set LDFLAGS and LIBS in
makefiles", 2022-11-30) / PR netblue30#5504.

config.mk.in

@@ -76,6 +76,7 @@ MANFLAGS = \
 # https://www.gnu.org/software/automake/manual/1.16.5/html_node/User-Variables.html
 CC=@CC@
 CFLAGS=@CFLAGS@
+CPPFLAGS=@CPPFLAGS@
 LDFLAGS=@LDFLAGS@
 
 # Project variables

configure

@@ -5308,6 +5308,7 @@ cat <<EOF
 Compile options:
    CC: $CC
    CFLAGS: $CFLAGS
+   CPPFLAGS: $CPPFLAGS
    LDFLAGS: $LDFLAGS
    EXTRA_CFLAGS: $EXTRA_CFLAGS
    EXTRA_LDFLAGS: $EXTRA_LDFLAGS

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

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)

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