configure: clean up plugin option handling

Keep together all the conditions that lead to disabling plugins, and
remove now-dead code.

Since the option was not in SKIP_OPTIONS, it was present twice in
the help message, both from configure and from meson-buildoptions.sh.
Remove the duplication and take the occasion to document the option as
autodetected, which it is.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -377,7 +377,6 @@ fi
 
 case $targetos in
 windows)
-  plugins="no"
   pie="no"
 ;;
 haiku)
@@ -741,7 +740,6 @@ for opt do
       default_cflags='-O0 -g'
   ;;
   --disable-tcg) tcg="disabled"
-                 plugins="no"
   ;;
   --enable-tcg) tcg="enabled"
   ;;
@@ -778,11 +776,7 @@ for opt do
   ;;
   --enable-download) download="enabled"; git_submodules_action=update;
   ;;
-  --enable-plugins) if test "$targetos" = "windows"; then
-                        error_exit "TCG plugins not currently supported on Windows platforms"
-                    else
-                        plugins="yes"
-                    fi
+  --enable-plugins) plugins="yes"
   ;;
   --disable-plugins) plugins="no"
   ;;
@@ -808,11 +802,6 @@ then
     git_submodules_action="validate"
 fi
 
-# test for any invalid configuration combinations
-if test "$plugins" = "yes" -a "$tcg" = "disabled"; then
-    error_exit "Can't enable plugins on non-TCG builds"
-fi
-
 if ! test -f "$source_path/subprojects/keycodemapdb/README" \
     && test "$download" = disabled
 then
@@ -923,8 +912,6 @@ Advanced options (experts only):
   --enable-debug           enable common debug build options
   --disable-werror         disable compilation abort on warning
   --cpu=CPU                Build for host CPU [$cpu]
-  --enable-plugins
-                           enable plugins via shared library loading
   --disable-containers     don't use containers for cross-building
   --container-engine=TYPE  which container engine to use [$container_engine]
   --gdb=GDB-path           gdb to use for gdbstub tests [$gdb_bin]
@@ -1066,15 +1053,27 @@ if test "$targetos" = "bogus"; then
     error_exit "Unrecognized host OS (uname -s reports '$(uname -s)')"
 fi
 
+# test for any invalid configuration combinations
+if test "$targetos" = "windows"; then
+  if test "$plugins" = "yes"; then
+    error_exit "TCG plugins not currently supported on Windows platforms"
+  fi
+  plugins="no"
+fi
+if test "$tcg" = "disabled" ; then
+  if test "$plugins" = "yes"; then
+    error_exit "Can't enable plugins on non-TCG builds"
+  fi
+  plugins="no"
+fi
 if test "$static" = "yes" ; then
   if test "$plugins" = "yes"; then
     error_exit "static and plugins are mutually incompatible"
-  else
-    plugins="no"
   fi
+  plugins="no"
 fi
-test "$plugins" = "" && plugins=yes
-if test "$plugins" = "yes"; then
+if test "$plugins" != "no"; then
+  plugins=yes
   subdirs="$subdirs contrib/plugins"
 fi
 
@@ -1717,10 +1716,6 @@ mkdir -p tests/tcg
 echo "# Automatically generated by configure - do not modify" > $config_host_mak
 echo "SRC_PATH=$source_path" >> $config_host_mak
 
-if test "$plugins" = "yes" ; then
-    echo "CONFIG_PLUGIN=y" >> $config_host_mak
-fi
-
 tcg_tests_targets=
 for target in $target_list; do
   arch=${target%%-*}