fix(build): keep the PCH-only flags in one variable so the opt-out can't drift

1f121a3 added -Winvalid-pch alongside -fpch-preprocess but left
makefiles/fppinit.mk filtering only the literal -fpch-preprocess, so the four
FPPINIT objects -- which deliberately opt out of the PCH -- went on carrying a
PCH-only warning flag.  It is inert today because none of those TUs include
fpp-pch.h, so gcc never goes looking for a .gch, but it would start warning the
moment one of them did, on exactly the objects that asked not to use the PCH.

Two hand-maintained copies of the same list drifting apart is what broke the PCH
to begin with, so name the list once: PCH_CFLAGS is defined where the flags are
added and filtered by reference where they are removed.

Verified from make -n on 32-bit armhf and aarch64:

    normal object     -fpch-preprocess yes   -Winvalid-pch yes   -g1 yes
    FPPINIT object                    no                   no        no
    fpp-pch.h.gch                    yes                  yes       yes

Full build otherwise unchanged: 177.7s, 204 objects, rc=0, and zero
-Winvalid-pch warnings.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: dkulp

src/Makefile

@@ -99,7 +99,7 @@ fppversion_defines.h: fppversion.c
 ifeq '$(findstring clang,$(CXXCOMPILER))' ''
 # Target-specific variables are inherited by a target's prerequisites, and
 # $(PCH_FILE) is a prerequisite of every %.o -- including the fppinit objects,
-# which do "CFLAGS := $(filter-out -fpch-preprocess -g1,$(CFLAGS))".  fppinit is
+# which do "CFLAGS := $(filter-out $(PCH_CFLAGS) -g1,$(CFLAGS))".  fppinit is
 # prepended to TARGETS, so it is built first and its objects are always what
 # pulls the shared .gch in, which means the .gch ends up built with flags that
 # no translation unit ever uses.  gcc then silently discards it for every TU

src/makefiles/common/setup.mk

@@ -129,12 +129,18 @@ ifeq '$(findstring clang,$(CXXCOMPILER))' ''
     # Common CFLAGS
 ifeq ($(DISTRIBUTED_COMPILE),)
     PCH_FILE=fpp-pch.h.gch
+	# Flags that mean something only to a compile that consumes the PCH.  Objects
+	# which deliberately opt out filter this same variable back out rather than
+	# repeating its contents, so the two lists cannot drift apart -- see
+	# makefiles/fppinit.mk.
+	#
 	# -Winvalid-pch: a .gch gcc decides it cannot use is discarded SILENTLY, and
 	# the build then re-parses the whole of fpp-pch.h in every TU -- slower than
 	# having no PCH at all, with nothing in the output to say so. That state went
 	# unnoticed for two months (see the note on the fpp-pch.h.gch rule in
 	# ../Makefile). This makes the next one loud instead: one warning per TU.
-	CFLAGS+=-fpch-preprocess -Winvalid-pch
+	PCH_CFLAGS := -fpch-preprocess -Winvalid-pch
+	CFLAGS+=$(PCH_CFLAGS)
 else
 	CFLAGS+=-DNOPCH
 endif

src/makefiles/fppinit.mk

@@ -16,7 +16,11 @@ LIBS_fppinit += \
 FPPINIT_DEPS=libfpp_capeutils.so
 endif
 
-boot/FPPINIT.o  boot/FPPINIT_Config.o  boot/FPPINIT_Network.o  boot/FPPINIT_Audio.o: CFLAGS := $(filter-out -fpch-preprocess -g1,$(CFLAGS))
+# These objects do not use the PCH, so drop the PCH-only flags ($(PCH_CFLAGS),
+# defined next to where they are added in makefiles/common/setup.mk) along with
+# -g1.  Note this is inherited by these targets' prerequisites: see the comment
+# on the fpp-pch.h.gch rule in ../Makefile for why that matters.
+boot/FPPINIT.o  boot/FPPINIT_Config.o  boot/FPPINIT_Network.o  boot/FPPINIT_Audio.o: CFLAGS := $(filter-out $(PCH_CFLAGS) -g1,$(CFLAGS))
 
 
 fppinit: $(OBJECTS_fppinit)  $(FPPINIT_DEPS)