perf(build): resolve pkg-config once at parse time, not once per recipe

CFLAGS is a recursively-expanded variable, so "CFLAGS += $(shell pkg-config
...)" appends the shell call itself rather than its result, and make re-runs it
every time CFLAGS is expanded -- which is once per compile recipe.  Three
helpers were wired up that way (gstreamer, libdrm, and GraphicsMagick++-config
on generic-Linux builds), so a build forked them several hundred times to
recompute a string that never changes.

Capture each into a simply-expanded variable and append that instead.  The
resulting compile lines are unchanged token for token; only the number of forks
differs.

    pkg-config invocations per build     406 -> 3
    make -n over a full build, 1-core armhf     11.5s -> 0.9s
    make -n over a full build, 4-core aarch64    0.8s -> 0.2s

The cost is paid on every make invocation, so it is proportionally largest on
no-op and incremental builds.

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

Author: dkulp

src/makefiles/fpp_so.mk

@@ -160,16 +160,25 @@ LIBS_fpp_so += \
     $(LIBS_GPIO_ADDITIONS)
 
 # GStreamer support
+#
+# Resolve pkg-config with := so it runs once while the makefiles are read.
+# CFLAGS is a recursively-expanded variable, so a bare "CFLAGS += $(shell ...)"
+# appends the shell call itself rather than its result, and make then re-runs
+# pkg-config every time CFLAGS is expanded -- which is once per compile recipe,
+# several hundred forks per build on boards that can least afford them.
 ifneq ($(wildcard /usr/include/gstreamer-1.0/gst/gst.h),)
-CFLAGS += $(shell pkg-config --cflags gstreamer-1.0 gstreamer-app-1.0 gstreamer-net-1.0)
-LIBS_fpp_so += $(shell pkg-config --libs gstreamer-1.0 gstreamer-app-1.0 gstreamer-net-1.0)
+GSTREAMER_CFLAGS := $(shell pkg-config --cflags gstreamer-1.0 gstreamer-app-1.0 gstreamer-net-1.0)
+GSTREAMER_LIBS := $(shell pkg-config --libs gstreamer-1.0 gstreamer-app-1.0 gstreamer-net-1.0)
+CFLAGS += $(GSTREAMER_CFLAGS)
+LIBS_fpp_so += $(GSTREAMER_LIBS)
 endif
 
 # DRM/KMS. fpp.cpp and framebuffer/KMSFrameBuffer.h enable the KMS code on
 # __has_include(<xf86drm.h>), so the flags have to key off that same header --
 # xf86drm.h itself pulls in <drm.h>, which only resolves with libdrm's -I.
 ifneq ($(wildcard /usr/include/xf86drm.h),)
-CFLAGS += $(shell pkg-config --cflags libdrm)
+LIBDRM_CFLAGS := $(shell pkg-config --cflags libdrm)
+CFLAGS += $(LIBDRM_CFLAGS)
 LIBS_fpp_so += -ldrm
 endif
 

src/makefiles/platform/linux.mk

@@ -35,7 +35,11 @@ ifeq '$(ISLINUX)' '1'
 # do something Linux-y
 
 ifeq ($(wildcard /usr/include/Magick++.h),)
-CFLAGS += $(shell GraphicsMagick++-config --cppflags)
+# := so the helper runs once at parse time rather than on every expansion of the
+# recursively-expanded CFLAGS, i.e. once per compile recipe. See the pkg-config
+# note in makefiles/fpp_so.mk.
+GMAGICK_CFLAGS := $(shell GraphicsMagick++-config --cppflags)
+CFLAGS += $(GMAGICK_CFLAGS)
 endif
 
 ifneq ($(wildcard /etc/fpp/container),)