build: Make build/installation of su and its support files optional

Enabled by default
This is necessary because coreutils and util-linux can also provide su

Signed-off-by: Lars Wendler <polynomial-c@gentoo.org>

etc/pam.d/Makefile.am

@@ -6,8 +6,7 @@ pamd_files = \
 	chsh \
 	groupmems \
 	login \
-	passwd \
-	su
+	passwd
 
 pamd_acct_tools_files = \
 	chage \
@@ -29,4 +28,8 @@ pamd_DATA += $(pamd_acct_tools_files)
 endif
 endif
 
+if WITH_SU
+pamd_files += su
+endif
+
 EXTRA_DIST = $(pamd_files) $(pamd_acct_tools_files)