autoconf: use ax_compute_relative_paths

configure.ac

@@ -4773,14 +4773,10 @@ fi
 SDL_STATIC_LIBS="$EXTRA_LDFLAGS"
 
 dnl Calculate the location of the prefix, relative to the cmake folder
-eval pkg_prefix=$prefix
-eval pkg_cmakedir=$libdir/cmake/SDL2
-cmake_prefix_relpath="$(echo -n "$pkg_cmakedir" | sed -E "s#^$pkg_prefix##" | sed -E "s#[A-Za-z0-9_ .-]+#..#g" )"
-AC_SUBST([cmake_prefix_relpath])
-
 dnl Calculate the location of the prefix, relative to bindir
-eval pkg_bindir=$bindir
-bin_prefix_relpath="$(echo -n "pkg_bindir" | sed -E "s#^$pkg_prefix##" | sed -E "s#[A-Za-z0-9_ .-]+#..#g" )"
+pkg_cmakedir='$libdir/cmake/SDL2'
+AX_COMPUTE_RELATIVE_PATHS([pkg_cmakedir:prefix:cmake_prefix_relpath bindir:prefix:bin_prefix_relpath])
+AC_SUBST([cmake_prefix_relpath])
 AC_SUBST([bin_prefix_relpath])
 
 dnl Expand the cflags and libraries needed by apps using SDL