Rename _SO_LIB to SO_LIB as it is now used beyond Makefile_plugin.common

Makefile_plugin.common

diff --git a/Makefile_plugin.common b/Makefile_plugin.common
index 45c2640..bfa9809 100644
--- a/Makefile_plugin.common
+++ b/Makefile_plugin.common
@@ -101,14 +101,14 @@
 
 # Shared library
 
-_SO_LIB := $(BUILD_DIR)/$(NAME).so
-_ALL_BUILD_SUBDIRS += $(abspath $(dir $(_SO_LIB)))
+SO_LIB := $(BUILD_DIR)/$(NAME).so
+_ALL_BUILD_SUBDIRS += $(abspath $(dir $(SO_LIB)))
 
-$(_SO_LIB): $(_ALL_OBJECTS) $(_MAKEFILES) | $(abspath $(dir $(_SO_LIB)))
+$(SO_LIB): $(_ALL_OBJECTS) $(_MAKEFILES) | $(abspath $(dir $(SO_LIB)))
 	$(CXX) $(CXXFLAGS) $(LDFLAGS) -shared -o $@ $(_ALL_OBJECTS) $(LDLIBS)
 
 .PHONY: $(NAME).so
-$(NAME).so: $(_SO_LIB)
+$(NAME).so: $(SO_LIB)
 
 # Tests
 
@@ -125,8 +125,8 @@
 
 # Installation
 
-$(YOSYS_PLUGINS_DIR)/$(NAME).so: $(_SO_LIB) | $(YOSYS_PLUGINS_DIR)
-	install -D $(_SO_LIB) $@
+$(YOSYS_PLUGINS_DIR)/$(NAME).so: $(SO_LIB) | $(YOSYS_PLUGINS_DIR)
+	install -D $(SO_LIB) $@
 
 .PHONY: install_plugin
 install_plugin: $(YOSYS_PLUGINS_DIR)/$(NAME).so
@@ -151,4 +151,3 @@
 
 $(PMGEN_PY):
 	@$(MAKE) -C $(TOP_DIR) pmgen.py
-

systemverilog-plugin/Makefile

diff --git a/systemverilog-plugin/Makefile b/systemverilog-plugin/Makefile
index 0e2f11f..25942f7 100644
--- a/systemverilog-plugin/Makefile
+++ b/systemverilog-plugin/Makefile
@@ -42,7 +42,7 @@
 $(BUILD_DIR)/.$(NAME)-deps-test:
 	$(PKG_CONFIG_INVOKE) --cflags Surelog
 
-${_SO_LIB}: | $(BUILD_DIR)/.$(NAME)-deps-test
+${SO_LIB}: | $(BUILD_DIR)/.$(NAME)-deps-test
 
 CXXFLAGS += -std=c++17 -Wall -W -Wextra \
             -Wno-deprecated-declarations \