Fix comparison operator in test on freebsd

Compare using = instead of ==.
Comparison with == works OK on Fedora 29 but fails on Ubuntu 23.10
with the following message:
./configure: 6792: test: linux: unexpected operator
According to the stackexchange the official syntax is a to use =
but sometimes the == may also work.
This comparison is introduced in commit 24d18ca.
Thanks to angelaschmid.

mythtv/configure

@@ -6789,7 +6789,7 @@ fi
 # incompatible function pointer waring has been promoted to an error.
 # Demoting it back to a warning allows the builds to work again.
 # https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2023-March/307515.html
-if test $target_os == "freebsd" &&
+if test $target_os = "freebsd" &&
 	enabled clangxx &&
 	test $clangxx_major -ge 16; then
    ffmpeg_extra_cflags="$ffmpeg_extra_cflags -Wno-error=incompatible-function-pointer-types"