multiprocess: move feature to meson_options.txt

While at it, improve the description of the feature in the summary and
help message.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index c79cb20..d38620d 100644
--- a/meson.build
+++ b/meson.build
@@ -157,6 +157,11 @@
   error('Multipath is supported only on Linux')
 endif
 
+if targetos != 'linux' and get_option('multiprocess').enabled()
+  error('Multiprocess QEMU is supported only on Linux')
+endif
+multiprocess_allowed = targetos == 'linux' and not get_option('multiprocess').disabled()
+
 m = cc.find_library('m', required: false)
 util = cc.find_library('util', required: false)
 winmm = []
@@ -1228,7 +1233,7 @@
   (have_virtfs ? ['CONFIG_VIRTFS=y'] : []) + \
   ('CONFIG_LINUX' in config_host ? ['CONFIG_LINUX=y'] : []) + \
   ('CONFIG_PVRDMA' in config_host ? ['CONFIG_PVRDMA=y'] : []) + \
-  ('CONFIG_MULTIPROCESS_ALLOWED' in config_host ? ['CONFIG_MULTIPROCESS_ALLOWED=y'] : [])
+  (multiprocess_allowed ? ['CONFIG_MULTIPROCESS_ALLOWED=y'] : [])
 
 ignored = [ 'TARGET_XML_FILES', 'TARGET_ABI_DIR', 'TARGET_ARCH' ]
 
@@ -2538,6 +2543,7 @@
 summary_info += {'target list':       ' '.join(target_dirs)}
 if have_system
   summary_info += {'default devices':   get_option('default_devices')}
+  summary_info += {'out of process emulation': multiprocess_allowed}
 endif
 summary(summary_info, bool_yn: true, section: 'Targets and accelerators')
 
@@ -2658,7 +2664,6 @@
 summary_info += {'libdaxctl support': config_host.has_key('CONFIG_LIBDAXCTL')}
 summary_info += {'libudev':           libudev.found()}
 summary_info += {'FUSE lseek':        fuse_lseek.found()}
-summary_info += {'Multiprocess QEMU': config_host.has_key('CONFIG_MULTIPROCESS_ALLOWED')}
 summary(summary_info, bool_yn: true, section: 'Dependencies')
 
 if not supported_cpus.contains(cpu)