diff --git a/Makefile b/Makefile
index 1f36a06..b324b9a 100644
--- a/Makefile
+++ b/Makefile
@@ -36,6 +36,27 @@
 generated_files: tests/generated_files
 generated_files: visualc_files
 
+# Set GEN_FILES to the empty string to disable dependencies on generated
+# source files. Then `make generated_files` will only build files that
+# are missing, it will not rebuilt files that are present but out of date.
+# This is useful, for example, if you have a source tree where
+# `make generated_files` has already run and file timestamps reflect the
+# time the files were copied or extracted, and you are now in an environment
+# that lacks some of the necessary tools to re-generate the files.
+# If $(GEN_FILES) is non-empty, the generated source files' dependencies
+# are treated ordinarily, based on file timestamps.
+GEN_FILES ?= yes
+
+# In dependencies where the target is a configuration-independent generated
+# file, use `TARGET: $(gen_file_dep) DEPENDENCY1 DEPENDENCY2 ...`
+# rather than directly `TARGET: DEPENDENCY1 DEPENDENCY2 ...`. This
+# enables the re-generation to be turned off when GEN_FILES is disabled.
+ifdef GEN_FILES
+gen_file_dep =
+else
+gen_file_dep = |
+endif
+
 .PHONY: visualc_files
 VISUALC_FILES = visualc/VS2013/mbedTLS.sln visualc/VS2013/mbedTLS.vcxproj
 # TODO: $(app).vcxproj for each $(app) in programs/
@@ -45,10 +66,10 @@
 # present before it runs. It doesn't matter if the files aren't up-to-date,
 # they just need to be present.
 $(VISUALC_FILES): | library/generated_files
-$(VISUALC_FILES): scripts/generate_visualc_files.pl
-$(VISUALC_FILES): scripts/data_files/vs2013-app-template.vcxproj
-$(VISUALC_FILES): scripts/data_files/vs2013-main-template.vcxproj
-$(VISUALC_FILES): scripts/data_files/vs2013-sln-template.sln
+$(VISUALC_FILES): $(gen_file_dep) scripts/generate_visualc_files.pl
+$(VISUALC_FILES): $(gen_file_dep) scripts/data_files/vs2013-app-template.vcxproj
+$(VISUALC_FILES): $(gen_file_dep) scripts/data_files/vs2013-main-template.vcxproj
+$(VISUALC_FILES): $(gen_file_dep) scripts/data_files/vs2013-sln-template.sln
 # TODO: also the list of .c and .h source files, but not their content
 $(VISUALC_FILES):
 	echo "  Gen   $@ ..."