configure: Warn about deprecated hosts

We plan to drop support in a future QEMU release for host OSes
and host architectures for which we have no test machine where
we can build and run tests. For the 2.9 release, make configure
print a warning if it is run on such a host, so that the user
has some warning of the plans and can volunteer to help us
maintain the port if they need it to continue to function.

This commit flags up as deprecated the CPU architectures:
* ia64
* sparc
* anything which we don't have a TCG port for
(and which was presumably using TCI)
and the OSes:
* GNU/kFreeBSD
* DragonFly BSD
* NetBSD
* OpenBSD
* Solaris
* AIX
* Haiku

It also makes entirely unrecognized host OS strings be
rejected rather than treated as if they were Linux (which
likely never worked).

Backports commit 898be3e0415c6d614395c087ef1e91210797cda7 from qemu

qemu/configure

@@ -178,6 +178,9 @@ tcg="yes"
 cpuid_h="no"
 avx2_opt="no"
 
+supported_cpu="no"
+supported_os="no"
+
 # parse CC options first
 for opt do
   optarg=$(expr "x$opt" : 'x[^=]*=\(.*\)')
@@ -356,23 +359,32 @@ ARCH=
 # Normalise host CPU name and set ARCH.
 # Note that this case should only have supported host CPUs, not guests.
 case "$cpu" in
-  ppc|ppc64|s390|s390x|sparc64|x32)
+  ppc|ppc64|s390|s390x|x32)
+    cpu="$cpu"
+    supported_cpu="yes"
+  ;;
+  sparc64)
     cpu="$cpu"
   ;;
   i386|i486|i586|i686|i86pc|BePC)
     cpu="i386"
+    supported_cpu="yes"
   ;;
   x86_64|amd64)
     cpu="x86_64"
+    supported_cpu="yes"
   ;;
   armv*b|armv*l|arm)
     cpu="arm"
+    supported_cpu="yes"
   ;;
   aarch64|aarch64eb)
     cpu="aarch64"
+    supported_cpu="yes"
   ;;
   mips*)
     cpu="mips"
+    supported_cpu="yes"
   ;;
   sparc|sun4[cdmuv])
     cpu="sparc"
@@ -391,6 +403,7 @@ fi
 case $targetos in
 MINGW32*)
   mingw32="yes"
+  supported_os="yes"
 ;;
 GNU/kFreeBSD)
   bsd="yes"
@@ -400,6 +413,7 @@ FreeBSD)
   make="${MAKE-gmake}"
   # needed for kinfo_getvmmap(3) in libutil.h
   LIBS="-lutil $LIBS"
+  supported_os="yes"
 ;;
 DragonFly)
   bsd="yes"
@@ -424,6 +438,7 @@ Darwin)
   # Disable attempts to use ObjectiveC features in os/object.h since they
   # won't work when we're compiling with gcc as a C compiler.
   QEMU_CFLAGS="-DOS_OBJECT_USE_OBJC=0 $QEMU_CFLAGS"
+  supported_os="yes"
 ;;
 SunOS)
   solaris="yes"
@@ -442,8 +457,12 @@ Haiku)
   QEMU_CFLAGS="-DB_USE_POSITIVE_POSIX_ERRORS $QEMU_CFLAGS"
   LIBS="-lposix_error_mapper -lnetwork $LIBS"
 ;;
-*)
+Linux)
   linux="yes"
+  supported_os="yes"
+;;
+*)
+  error_exit "Unsupported host OS $targetos"
 ;;
 esac
 
@@ -1288,6 +1307,32 @@ if test "$tcg" = "yes" ; then
 fi
 echo "avx2 optimization $avx2_opt"
 
+if test "$supported_cpu" = "no"; then
+    echo
+    echo "WARNING: SUPPORT FOR THIS HOST CPU WILL GO AWAY IN FUTURE RELEASES!"
+    echo
+    echo "CPU host architecture $cpu support is not currently maintained."
+    echo "The QEMU project intends to remove support for this host CPU in"
+    echo "a future release if nobody volunteers to maintain it and to"
+    echo "provide a build host for our continuous integration setup."
+    echo "configure has succeeded and you can continue to build, but"
+    echo "if you care about QEMU on this platform you should contact"
+    echo "us upstream at qemu-devel@nongnu.org."
+fi
+
+if test "$supported_os" = "no"; then
+    echo
+    echo "WARNING: SUPPORT FOR THIS HOST OS WILL GO AWAY IN FUTURE RELEASES!"
+    echo
+    echo "CPU host OS $targetos support is not currently maintained."
+    echo "The QEMU project intends to remove support for this host CPU in"
+    echo "a future release if nobody volunteers to maintain it and to"
+    echo "provide a build host for our continuous integration setup."
+    echo "configure has succeeded and you can continue to build, but"
+    echo "if you care about QEMU on this platform you should contact"
+    echo "us upstream at qemu-devel@nongnu.org."
+fi
+
 config_host_mak="config-host.mak"
 
 echo "# Automatically generated by configure - do not modify" > $config_host_mak