Allow the use of GNU C++-compatible compilers that are not called "g++".

https://github.com/uniconproject/unicon/issues/559
--- configure.ac.orig	2020-12-20 23:08:43.000000000 -0600
+++ configure.ac	2026-03-30 02:56:12.000000000 -0500
@@ -532,7 +532,7 @@
           CHECK_OPENGL()
        fi
        # A C++ compiler must be present and usable
-       if test "x$CXX" = "xg++" -a "x$GXX" = "xyes"; then
+       if test "x$GXX" = "xyes"; then
          CHECK_FTGL()
        else
          echo "FTGL: missing or untrusted C++; CXX is $CXX and GXX is $GXX"
@@ -636,7 +636,7 @@
 
 if ! test "x$voip" = "xno"; then
     # A C++ compiler must be present and usable
-    if test "$CXX" = "g++" -a "$GXX" = "yes"; then
+    if test "$GXX" = "yes"; then
       CHECK_VOICE()
     else
          echo "VOIP: missing or untrusted C++; CXX is $CXX and GXX is $GXX"