diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9ed7e56e8..e881dbd1a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -834,9 +834,8 @@ set_target_properties(hb-ot-tag PROPERTIES COMPILE_FLAGS "-DMAIN")
 if (UNIX OR MINGW)
   if (BUILD_SHARED_LIBS)
     # generate harfbuzz.def after build completion
-    string(REPLACE ";" " " space_separated_headers "${project_headers}")
     add_custom_command(TARGET harfbuzz POST_BUILD
-      COMMAND ${CMAKE_COMMAND} -E env "headers=${space_separated_headers}" python ${PROJECT_SOURCE_DIR}/src/gen-def.py ${PROJECT_BINARY_DIR}/harfbuzz.def
+      COMMAND "${PYTHON_EXECUTABLE}" ${PROJECT_SOURCE_DIR}/src/gen-def.py ${PROJECT_BINARY_DIR}/harfbuzz.def ${project_headers}
       WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/src)
 
     add_test(NAME check-static-inits.sh
diff --git a/src/Makefile.am b/src/Makefile.am
index 9d5662e4a..6dfec3bfa 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -277,13 +277,13 @@ endif
 check: $(DEF_FILES) # For check-symbols.sh
 CLEANFILES += $(DEF_FILES)
 harfbuzz.def: $(HBHEADERS) $(HBNODISTHEADERS)
-	$(AM_V_GEN) headers="$^" $(srcdir)/gen-def.py "$@"
+	$(AM_V_GEN) $(srcdir)/gen-def.py "$@" $^
 harfbuzz-subset.def: $(HB_SUBSET_headers)
-	$(AM_V_GEN) headers="$^" $(srcdir)/gen-def.py "$@"
+	$(AM_V_GEN) $(srcdir)/gen-def.py "$@" $^
 harfbuzz-icu.def: $(HB_ICU_headers)
-	$(AM_V_GEN) headers="$^" $(srcdir)/gen-def.py "$@"
+	$(AM_V_GEN) $(srcdir)/gen-def.py "$@" $^
 harfbuzz-gobject.def: $(HB_GOBJECT_headers)
-	$(AM_V_GEN) headers="$^" $(srcdir)/gen-def.py "$@"
+	$(AM_V_GEN) $(srcdir)/gen-def.py "$@" $^
 
 
 GENERATORS = \
diff --git a/src/gen-def.py b/src/gen-def.py
index 9a997d65a..ba39eaae5 100755
--- a/src/gen-def.py
+++ b/src/gen-def.py
@@ -4,8 +4,14 @@ from __future__ import print_function, division, absolute_import
 
 import io, os, re, sys
 
+if len (sys.argv) < 3:
+	sys.exit("usage: gen-def.py harfbuzz.def hb.h [hb-blob.h hb-buffer.h ...]")
+
+output_file = sys.argv[1]
+header_paths = sys.argv[2:]
+
 headers_content = []
-for h in os.environ["headers"].split (' '):
+for h in header_paths:
 	if h.endswith (".h"):
 		with io.open (h, encoding='utf-8') as f: headers_content.append (f.read ())
 
@@ -13,7 +19,7 @@ result = """EXPORTS
 %s
 LIBRARY lib%s-0.dll""" % (
 	"\n".join (sorted (re.findall (r"^hb_\w+(?= \()", "\n".join (headers_content), re.M))),
-	sys.argv[1].replace ('.def', '')
+	output_file.replace ('.def', '')
 )
 
-with open (sys.argv[1], "w") as f: f.write (result)
+with open (output_file, "w") as f: f.write (result)