Call inner make in sane way, with error propagation. So build will fail on errors
--- a/Makefile	2024-04-01 09:38:48.800355255 +0000
+++ b/Makefile	2024-04-01 09:40:12.944908086 +0000
@@ -2,7 +2,7 @@
 TMP=.
 
 all: comment
-	cd src20; make; cd ..
+	$(MAKE) -C src20
 
 comment:
 	@echo
@@ -12,12 +12,12 @@
 	@echo 
 
 install: comment
-	cd src20; make install; cd ..
+	$(MAKE) -C src20 install
 
 distclean: clean
 
 clean:
-	cd src20; make clean; cd ..
+	$(MAKE) -C src20 clean
 	rm -f gkrellsun*.zip gkrellsun*.tar.gz
 
 dist:
Don't call gcc directly, remove -O2 -Wall flags, respect user's pkg-config
--- a/src20/Makefile
+++ b/src20/Makefile
@@ -1,8 +1,8 @@
 PACKAGE ?= gkrellsun
 
-GTK_CONFIG ?=pkg-config gtk+-2.0
-GTK_INCLUDE ?= `pkg-config gtk+-2.0 --cflags`
-GTK_LIB ?= `pkg-config gtk+-2.0 --libs`
+PKG_CONFIG ?= pkg-config
+GTK_INCLUDE ?= $(shell ${PKG_CONFIG} gtk+-2.0 --cflags)
+GTK_LIB ?= $(shell ${PKG_CONFIG} gtk+-2.0 --libs)
 
 INSTALL ?= install
 
@@ -11,7 +11,7 @@ INSTALLDIR ?= $(DESTDIR)$(PREFIX)
 PLUGINDIR ?= $(INSTALLDIR)/lib/gkrellm2/plugins
 LOCALEDIR ?= $(INSTALLDIR)/share/locale
 
-FLAGS = -O2 -Wall -fPIC $(GTK_INCLUDE)
+FLAGS = -fPIC $(GTK_INCLUDE)
 #FLAGS = -g -Wall -fPIC $(GTK_INCLUDE)
 LIBS = $(GTK_LIB)
 LFLAGS = -shared
@@ -25,7 +25,7 @@ endif
 FLAGS += -DPACKAGE="\"$(PACKAGE)\""
 export PACKAGE LOCALEDIR
 
-CC = gcc $(CFLAGS) $(FLAGS)
+CC = $(CC)
 
 OBJS = gkrellsun.o CalcEphem.o Moon.o MoonRise.o
 
@@ -37,10 +37,10 @@ all: gkrellsun.so
 
 gkrellsun.so: $(OBJS)
 	(cd po && ${MAKE})
-	$(CC) $(OBJS) -o gkrellsun.so $(LFLAGS) $(LIBS)
+	$(CC) $(CFLAGS) $(FLAGS) $(OBJS) -o gkrellsun.so $(LFLAGS) $(LIBS)
 
 suninfo: suninfo.o CalcEphem.o Moon.o MoonRise.o
-	$(CC) $^ -o suninfo -lm $(LIBS)
+	$(CC) $(CFLAGS) $(FLAGS) $^ -o suninfo -lm $(LIBS)
 
 clean:
 	rm -f *.o core *.so* *.bak *~
@@ -49,7 +49,8 @@ gkrellsun.o: gkrellsun.c $(IMAGES)
 
 $(OBJS): CalcEphem.h Moon.h MoonRise.h
 
-#%.o: %.c
+%.o: %.c
+	$(CC) $(CFLAGS) $(FLAGS) -c -o $@ $<
 
 install: gkrellsun.so
 	(cd po && ${MAKE} install )