Sent upstream by mail.

https://bugs.gentoo.org/920788
https://github.com/ImageMagick/ImageMagick/pull/6978

configure.ac: fixup LFS check for autoconf-2.72

For the 'other' case, we were using the value of ac_cv_sys_file_offset_bits for
-D_FILE_OFFSET_BITS to pass down into the Perl module build, but autoconf-2.72
drops the use of ac_cv_sys_file_offset_bits in cf09f48841b66fe76f606dd6018bb3a93242a7c9,
so this ends up defining '-D_FILE_OFFSET_BITS=' which then breaks the build.

I've just added a hack to preserve the old behaviour to avoid passing a blank
value.
--- a/configure.ac
+++ b/configure.ac
@@ -386,7 +386,7 @@ AC_FUNC_FSEEKO
 LFS_CPPFLAGS=''
 if test "$enable_largefile" != no
 then
-  if test "$ac_cv_sys_file_offset_bits" != 'no'
+  if test "$ac_cv_sys_file_offset_bits" != 'no' && test -n "$ac_cv_sys_file_offset_bits"
   then
     LFS_CPPFLAGS="$LFS_CPPFLAGS -D_FILE_OFFSET_BITS=$ac_cv_sys_file_offset_bits"
   fi