diff -up postgis-1.5.1/postgis/Makefile.pgxs.pgsql9 postgis-1.5.1/postgis/Makefile.pgxs
--- postgis-1.5.1/postgis/Makefile.pgxs.pgsql9	2010-01-12 11:33:49.000000000 +0100
+++ postgis-1.5.1/postgis/Makefile.pgxs	2011-04-03 13:45:33.972315988 +0200
@@ -12,6 +12,11 @@ datamoduledir = contrib
 docmoduledir = contrib
 endif
 
+# cope with PostgreSQL 9.0 build environment
+ifndef mkinstalldirs
+mkinstalldirs = $(MKDIR_P)
+endif
+
 install: all installdirs
 ifneq (,$(DATA)$(DATA_built))
 	@for file in $(addprefix $(srcdir)/, $(DATA)) $(DATA_built); do \