use macro for deps, unconditionally do AC_SUBSTs

AC_SUBSTs should be done in the correct block,
even if the main body of the macro is skipped.

Author: dimpase

build/pkgs/freetype/spkg-configure.m4

@@ -1,14 +1,9 @@
 SAGE_SPKG_CONFIGURE([freetype], [
-    AC_REQUIRE([SAGE_SPKG_CONFIGURE_LIBPNG])
-    AC_MSG_CHECKING([Installing libpng? ])
-    if test x$sage_spkg_install_libpng = xyes; then
-      AC_MSG_RESULT([yes; install freetype as well])
-      sage_spkg_install_freetype=yes
-    else
-      AC_MSG_RESULT([no])
+    SAGE_SPKG_DEPCHECK([libpng], [
       dnl freetype versions are libtool's ones, cf trac #30014
       PKG_CHECK_MODULES([FREETYPE], [freetype2 >= 16.1], [], [sage_spkg_install_freetype=yes])
-    fi
+    ])
+], [], [], [
     if test x$sage_spkg_install_freetype = xyes; then
       AC_SUBST(SAGE_FREETYPE_PREFIX, ['$SAGE_LOCAL'])
     else