configure: On Solaris, libpng requires libm.

configure.ac

diff --git a/configure.ac b/configure.ac
index 84ffddd..d71d92d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1201,7 +1201,7 @@
 dnl **** Check for libpng ****
 if test "$ac_cv_header_png_h" = "yes"
 then
-    WINE_CHECK_SONAME(png,png_create_read_struct,,,,[[libpng[[0-9]]*]])
+    WINE_CHECK_SONAME(png,png_create_read_struct,,,-lm,[[libpng[[0-9]]*]])
 fi
 WINE_WARNING_WITH(png,[test "x$ac_cv_lib_soname_png" = "x"],
                  [libpng development files not found, PNG won't be supported.])