Makefile: rebuild on flags changes

This invalidates all caches in case a flag like DEBUG=1 is specified on
the make command line.

.gitignore

@@ -2,5 +2,6 @@
 build
 __pycache__
 configure.mk
+flags.mk
 result
 gcroots

Makefile

@@ -253,6 +253,13 @@ graph.svg: Makefile
 # Rebuild everything if the makefile changed
 .EXTRA_PREREQS += Makefile
 
+# Rebuild everything if a flag changed
+.EXTRA_PREREQS += flags.mk
+
+# Store current make flags
+FLAGS=$(filter-out -%,$(MAKEFLAGS))
+$(shell test '$(FLAGS)' != "`cat flags.mk 2>&1`" && echo '$(FLAGS)' > flags.mk)
+
 # Search for source code in vendor/ subdirectory
 vpath %.f90 $(SRCDIR):$(SRCDIR)/vendor
 
@@ -275,7 +282,7 @@ $(OBJDIR)/%.d: %.f90 | $(OBJDIR)
 %.a : ; $(AR) $(ARFLAGS) $@ $?
 
 # Load the generated rules
-include $(DEPS)
+-include $(DEPS)
 
 # Compile a generic Fortran source
 $(OBJDIR)/%.o: %.f90 | $(OBJDIR) $(INCDIR)
@@ -287,4 +294,4 @@ $(INCDIR)/%.inc: $(SRCDIR)/%.awk | $(INCDIR)
 
 # Create directories
 $(DIRS):
-	mkdir -p '$@'
+	@mkdir -p '$@'