configure: Never use 'uname' to identify target OS

For a very long time we have used 'uname -s' as our fallback if
we don't identify the target OS using a compiler #define. This
obviously doesn't work for cross-compilation, and we've had
a comment suggesting we fix this in configure for a long time.
Since we now have an exhaustive list of which OSes we can run
on (thanks to commit 898be3e0415 making an unrecognized OS
be a fatal error), we know which ones we're missing.

Add check_define tests for the remaining OSes we support. The
defines checked are based on ones we already use in the codebase for
identifying the host OS (with the exception of GNU/kFreeBSD).
We can now set bogus_os immediately rather than doing it later.

We leave the comment about uname being bad untouched, since
there is still a use of it for the fallback for unrecognized
host CPU type.

Backports commit 951fedfceeda1b09ac8aa1f5263288b65e13caca from qemu

qemu/configure

@@ -290,8 +290,22 @@ elif check_define __sun__ ; then
   targetos='SunOS'
 elif check_define __HAIKU__ ; then
   targetos='Haiku'
+elif check_define __FreeBSD__ ; then
+  targetos='FreeBSD'
+elif check_define __FreeBSD_kernel__ && check_define __GLIBC__; then
+  targetos='GNU/kFreeBSD'
+elif check_define __DragonFly__ ; then
+  targetos='DragonFly'
+elif check_define __NetBSD__; then
+  targetos='NetBSD'
+elif check_define __APPLE__; then
+  targetos='Darwin'
 else
-  targetos=$(uname -s)
+  # This is a fatal error, but don't report it yet, because we
+  # might be going to just print the --help text, or it might
+  # be the result of a missing compiler.
+  targetos='bogus'
+  bogus_os='yes'
 fi
 
 # Some host OSes need non-standard checks for which CPU to use.
@@ -462,12 +476,6 @@ Linux)
   linux="yes"
   supported_os="yes"
 ;;
-*)
-  # This is a fatal error, but don't report it yet, because we
-  # might be going to just print the --help text, or it might
-  # be the result of a missing compiler.
-  bogus_os="yes"
-;;
 esac
 
 : ${make=${MAKE-make}}
@@ -713,7 +721,7 @@ if test "$bogus_os" = "yes"; then
     # the compiler works (so the results of the check_defines we used
     # to identify the OS are reliable), if we didn't recognize the
     # host OS we should stop now.
-    error_exit "Unrecognized host OS $targetos"
+    error_exit "Unrecognized host OS (uname -s reports '$(uname -s)')"
 fi
 
 gcc_flags="-Wold-style-declaration -Wold-style-definition -Wtype-limits"