ffbuild/library.mak: add the devprogs target

Commit a8968b1425 made the build and check targets depend on devprogs,
but no rule defines devprogs, so make build and make check stop with
"No rule to make target 'devprogs'". Define it like testprogs, from
the DEVPROGS of each library, and mark it phony.

Signed-off-by: lusoris <lusoris@proton.me>

Makefile

@@ -234,5 +234,5 @@ $(sort $(OUTDIRS)):
 # so this saves some time on slow systems.
 .SUFFIXES:
 
-.PHONY: all all-yes alltools build check config testprogs
+.PHONY: all all-yes alltools build check config devprogs testprogs
 .PHONY: *clean install* uninstall*

ffbuild/library.mak

@@ -156,3 +156,4 @@ $(TOOLS):     $(DEP_LIBS) $(SUBDIR)$($(CONFIG_SHARED:yes=S)LIBNAME)
 $(TESTPROGS) $(DEVPROGS): $(DEP_LIBS) $(SUBDIR)$(LIBNAME)
 
 testprogs: $(TESTPROGS)
+devprogs: $(DEVPROGS)