makefiles: deduplicate lib makefiles into so.mk

The following makefiles are nearly identical, except for the main target
name and for any extra headers that they might use:

* src/libpostexecseccomp/Makefile
* src/libtrace/Makefile
* src/libtracelog/Makefile

So move all of their (duplicated) code into a new src/so.mk file, and
add an include of src/so.mk, which leaves only variables, and the
includes of config.mk and src/so.mk in place.

With this commit, CFLAGS and LDFLAGS are only defined/changed in the
following files:

* config.mk.in
* src/common.mk
* src/so.mk

src/libpostexecseccomp/Makefile

@@ -6,23 +6,4 @@ TARGET = $(SO)
 
 MOD_HDRS = ../include/seccomp.h ../include/rundefs.h
 
-HDRS := $(sort $(wildcard *.h)) $(MOD_HDRS)
-SRCS := $(sort $(wildcard *.c)) $(MOD_SRCS)
-OBJS := $(SRCS:.c=.o) $(MOD_OBJS)
-CFLAGS += -ggdb $(HAVE_FATAL_WARNINGS) -O2 -DVERSION='"$(VERSION)"' -fstack-protector-all -D_FORTIFY_SOURCE=2 -fPIC -Wformat -Wformat-security
-LDFLAGS += -pie -fPIE -Wl,-z,relro -Wl,-z,now
-
-.PHONY: all
-all: $(TARGET)
-
-%.o : %.c $(HDRS) $(ROOT)/config.mk
-	$(CC) $(CFLAGS) $(INCLUDE) -c $< -o $@
-
-$(SO): $(OBJS) $(ROOT)/config.mk
-	$(CC) $(LDFLAGS) -shared -fPIC -z relro -o $@ $(OBJS) -ldl
-
-.PHONY: clean
-clean:; rm -fr $(OBJS) $(SO) *.plist
-
-.PHONY: distclean
-distclean: clean
+include $(ROOT)/src/so.mk

src/libtrace/Makefile

@@ -4,23 +4,4 @@ ROOT = ../..
 SO = libtrace.so
 TARGET = $(SO)
 
-HDRS := $(sort $(wildcard *.h)) $(MOD_HDRS)
-SRCS := $(sort $(wildcard *.c)) $(MOD_SRCS)
-OBJS := $(SRCS:.c=.o) $(MOD_OBJS)
-CFLAGS += -ggdb $(HAVE_FATAL_WARNINGS) -O2 -DVERSION='"$(VERSION)"' -fstack-protector-all -D_FORTIFY_SOURCE=2 -fPIC -Wformat -Wformat-security
-LDFLAGS += -pie -fPIE -Wl,-z,relro -Wl,-z,now
-
-.PHONY: all
-all: $(TARGET)
-
-%.o : %.c $(HDRS) $(ROOT)/config.mk
-	$(CC) $(CFLAGS) $(INCLUDE) -c $< -o $@
-
-$(SO): $(OBJS) $(ROOT)/config.mk
-	$(CC) $(LDFLAGS) -shared -fPIC -z relro -o $@ $(OBJS) -ldl
-
-.PHONY: clean
-clean:; rm -fr $(OBJS) $(SO) *.plist
-
-.PHONY: distclean
-distclean: clean
+include $(ROOT)/src/so.mk

src/libtracelog/Makefile

@@ -6,23 +6,4 @@ TARGET = $(SO)
 
 MOD_HDRS = ../include/rundefs.h
 
-HDRS := $(sort $(wildcard *.h)) $(MOD_HDRS)
-SRCS := $(sort $(wildcard *.c)) $(MOD_SRCS)
-OBJS := $(SRCS:.c=.o) $(MOD_OBJS)
-CFLAGS += -ggdb $(HAVE_FATAL_WARNINGS) -O2 -DVERSION='"$(VERSION)"' -fstack-protector-all -D_FORTIFY_SOURCE=2 -fPIC -Wformat -Wformat-security
-LDFLAGS += -pie -fPIE -Wl,-z,relro -Wl,-z,now
-
-.PHONY: all
-all: $(TARGET)
-
-%.o : %.c $(HDRS) $(ROOT)/config.mk
-	$(CC) $(CFLAGS) $(INCLUDE) -c $< -o $@
-
-$(SO): $(OBJS) $(ROOT)/config.mk
-	$(CC) $(LDFLAGS) -shared -fPIC -z relro -o $@ $(OBJS) -ldl
-
-.PHONY: clean
-clean:; rm -fr $(OBJS) $(SO) *.plist
-
-.PHONY: distclean
-distclean: clean
+include $(ROOT)/src/so.mk

src/so.mk

@@ -0,0 +1,25 @@
+# Common definitions for making shared objects.
+#
+# Note: $(ROOT)/config.mk must be included before this file.
+
+HDRS := $(sort $(wildcard *.h)) $(MOD_HDRS)
+SRCS := $(sort $(wildcard *.c)) $(MOD_SRCS)
+OBJS := $(SRCS:.c=.o) $(MOD_OBJS)
+
+CFLAGS += -ggdb $(HAVE_FATAL_WARNINGS) -O2 -DVERSION='"$(VERSION)"' -fstack-protector-all -D_FORTIFY_SOURCE=2 -fPIC -Wformat -Wformat-security
+LDFLAGS += -pie -fPIE -Wl,-z,relro -Wl,-z,now
+
+.PHONY: all
+all: $(TARGET)
+
+%.o : %.c $(HDRS) $(ROOT)/config.mk
+	$(CC) $(CFLAGS) $(INCLUDE) -c $< -o $@
+
+$(SO): $(OBJS) $(ROOT)/config.mk
+	$(CC) $(LDFLAGS) -shared -fPIC -z relro -o $@ $(OBJS) -ldl
+
+.PHONY: clean
+clean:; rm -fr $(OBJS) $(SO) *.plist
+
+.PHONY: distclean
+distclean: clean