--- a/acinclude.m4
+++ b/acinclude.m4
@@ -163,21 +163,21 @@
   libpng_prefix="")
 
 min_libpng_version=ifelse([$1], ,1.2.0,$1)
-tempLIBS="$LIBS"
-tempCFLAGS="$CFLAGS"
-if test x$libpng_prefix != x ; then
-  LIBPNG_LIBS="-L$libpng_prefix"
-  LIBPNG_CFLAGS="-I$libpng_prefix"
-fi
-LIBPNG_LIBS="$LIBPNG_LIBS -lpng -lm"
-LIBS="$LIBS $LIBPNG_LIBS"
-CFLAGS="$CFLAGS $LIBPNG_CFLAGS"
 
 AC_MSG_CHECKING(for libpng - version >= $min_libpng_version)
 
 dnl <--- disable for no user choice part #2
 if test x$enable_libpng != xno; then
 dnl --->
+  tempLIBS="$LIBS"
+  tempCFLAGS="$CFLAGS"
+  if test x$libpng_prefix != x ; then
+    LIBPNG_LIBS="-L$libpng_prefix"
+    LIBPNG_CFLAGS="-I$libpng_prefix"
+  fi
+  LIBPNG_LIBS="$LIBPNG_LIBS -lpng -lm"
+  LIBS="$LIBS $LIBPNG_LIBS"
+  CFLAGS="$CFLAGS $LIBPNG_CFLAGS"
 
   AC_RUN_IFELSE([AC_LANG_SOURCE([[
 #include <png.h>