tests/plugins: don't unconditionally add -Wpsabi

Not all compilers support the -Wpsabi (clang-9 in my case). To handle
this gracefully we pare back the shared build machinery so the
Makefile is relatively "standalone". We still take advantage of
config-host.mak as configure has done a bunch of probing for us but
that is it.

Fixes: bac8d222a
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200713200415.26214-6-alex.bennee@linaro.org>

configure

@@ -7115,6 +7115,9 @@ echo "GIT_UPDATE=$git_update" >> $config_host_mak
 
 echo "ARCH=$ARCH" >> $config_host_mak
 
+echo "GLIB_CFLAGS=$glib_cflags" >> $config_host_mak
+echo "GLIB_LDFLAGS=$glib_ldflags" >> $config_host_mak
+
 if test "$default_devices" = "yes" ; then
   echo "CONFIG_MINIKCONF_MODE=--defconfig" >> $config_host_mak
 else

tests/plugin/Makefile

@@ -1,9 +1,16 @@
+# -*- Mode: makefile -*-
+#
+# This Makefile example is fairly independent from the main makefile
+# so users can take and adapt it for their build. We only really
+# include config-host.mak so we don't have to repeat probing for
+# cflags that the main configure has already done for us.
+#
+
 BUILD_DIR := $(CURDIR)/../..
 
 include $(BUILD_DIR)/config-host.mak
-include $(SRC_PATH)/rules.mak
 
-$(call set-vpath, $(SRC_PATH)/tests/plugin)
+VPATH += $(SRC_PATH)/tests/plugin
 
 NAMES :=
 NAMES += bb
@@ -17,11 +24,18 @@ NAMES += lockstep
 
 SONAMES := $(addsuffix .so,$(addprefix lib,$(NAMES)))
 
-QEMU_CFLAGS += -fPIC -Wpsabi
-QEMU_CFLAGS += -I$(SRC_PATH)/include/qemu
+# The main QEMU uses Glib extensively so it's perfectly fine to use it
+# in plugins (which many example do).
+CFLAGS = $(GLIB_CFLAGS)
+CFLAGS += -fPIC
+CFLAGS += $(if $(findstring no-psabi,$(QEMU_CFLAGS)),-Wpsabi)
+CFLAGS += -I$(SRC_PATH)/include/qemu
 
 all: $(SONAMES)
 
+%.o: %.c
+	$(CC) $(CFLAGS) -c -o $@ $<
+
 lib%.so: %.o
 	$(CC) -shared -Wl,-soname,$@ -o $@ $^ $(LDLIBS)