configure: fix broken cross-compilation detection

Assume the build is native if the compiler for the host and the build
machine are the same.

For example, cbuild (Chimera Linux package builder) defines both the
host and the build compiler, regardless of native or cross build.

Also, that assumption in this case is consistent with GNU autotools
behaviour.

Author: mobin-2008

configure

@@ -450,6 +450,16 @@ for arg in "$@"; do
     esac
 done
 
+## Assume the build is native if both $CXX and $CXX_FOR_BUILD have the same value.
+# We use the value of $CXX_FOR_BUILD to detect cross-build. However, it is fairly common to set
+# both variables to the same compiler in a native build. For example, cbuild (Chimera Linux package
+# builder) sets both variables regardless of native or cross build. Also, GNU autotools assumes the
+# build is native in this case. Unset $CXX_FOR_BUILD now to be consistent with GNU autotools and
+# what people expect.
+if [ -n "${CXX_FOR_BUILD:-}" ] && [ "$CXX_FOR_BUILD" = "$CXX" ]; then
+    unset CXX_FOR_BUILD
+fi
+
 ## Check platform is specified for a cross-build
 if [ -n "${CXX_FOR_BUILD:-}" ]; then
     if [ -z "${PLATFORM:-}" ]; then