Add make target shared-gc

Allows building shared GC using `make shared-gc SHARED_GC=gc_impl`

.github/workflows/compilers.yml

@@ -263,13 +263,9 @@ jobs:
 
       - name: Build shared GC
         run: >
-          make probes.h &&
-          export RUBY_GC_LIBRARY=librubygc.so &&
-          echo "RUBY_GC_LIBRARY=$RUBY_GC_LIBRARY" >> $GITHUB_ENV &&
+          echo "RUBY_GC_LIBRARY=librubygc.gc_impl.so" >> $GITHUB_ENV &&
           mkdir ${{ matrix.entry.shared_gc_dir }} &&
-          gcc -I../src -I../src/include -I. -I.ext/include/x86_64-linux
-          -Wl,-undefined,dynamic_lookup -fPIC -g -O3 -shared
-          -o ${{ matrix.entry.shared_gc_dir }}/$RUBY_GC_LIBRARY ../src/gc_impl.c
+          make shared-gc SHARED_GC=gc_impl
         if: ${{ matrix.entry.shared_gc }}
 
       - name: Add to ext/Setup

common.mk

@@ -1906,6 +1906,18 @@ rewindable:
 
 HELP_EXTRA_TASKS = ""
 
+shared-gc: probes.h
+	$(Q) if test -z $(shared_gc_dir); then \
+		echo "You must configure with --with-shared-gc to use shared GC"; \
+		exit 1; \
+	elif test -z $(SHARED_GC); then \
+		echo "You must specify SHARED_GC with the GC to build"; \
+		exit 1; \
+	else \
+		echo generating $(shared_gc_dir)librubygc.$(SHARED_GC).$(SOEXT); \
+		$(LDSHARED) -I$(srcdir)/include -I$(srcdir) -I$(arch_hdrdir) -I. $(XDLDFLAGS) $(cflags) -fPIC -o $(shared_gc_dir)librubygc.$(SHARED_GC).$(SOEXT) $(srcdir)/$(SHARED_GC).c; \
+	fi
+
 help: PHONY
 	$(MESSAGE_BEGIN) \
 	"                Makefile of Ruby" \

template/Makefile.in

@@ -67,6 +67,8 @@ RUBY_VERSION_NAME = @RUBY_VERSION_NAME@
 UNIVERSAL_ARCHNAMES = @UNIVERSAL_ARCHNAMES@
 BUILTIN_BINARY = @X_BUILTIN_BINARY@
 
+shared_gc_dir = @shared_gc_dir@
+
 TESTUI = console
 TESTS =
 INSTALLDOC = @INSTALLDOC@

tool/m4/ruby_shared_gc.m4

@@ -33,4 +33,6 @@ AS_IF([test x"$shared_gc_dir" != x], [
 
     shared_gc_summary="no"
 ])
+
+AC_SUBST(shared_gc_dir, "${shared_gc_dir}")
 ])dnl