fix proj >= 7 detection
https://github.com/Xastir/Xastir/commit/6ee6348ca9ad8556e48d26d976f5d6709e8e31cb

Index: configure.ac
--- configure.ac.orig
+++ configure.ac
@@ -359,7 +359,7 @@ use_proj=yes
 AC_ARG_WITH(libproj,[  --without-libproj         Disable libproj features.],use_proj=$withval)
 if test "${use_proj}" = "yes"; then
   use_proj=no
-  AC_CHECK_LIB(proj, pj_init, use_proj=yes LIBS="$LIBS -lproj"
+  AC_CHECK_LIB(proj, proj_create_crs_to_crs, use_proj=yes LIBS="$LIBS -lproj"
     AC_DEFINE(HAVE_PROJ, 1, Define to 1 if you have the `libproj' library (-lproj). )
   )
 fi
