Set install_name for libtrufflerubytrampoline

* So macOS actually looks in rpath instead of ignoring it...

Author: eregon

src/main/c/cext-trampoline/Makefile

@@ -11,23 +11,32 @@ endif
 # If that is the case and fails the build, it might be better to use instead:
 # CC := $(GRAALVM_TOOLCHAIN_CC)
 
-# Same values as in RbConfig
+# Similar values as in RbConfig
 ifeq ($(OS),Darwin)
-CC := clang -fdeclspec
+CC := clang
+CFLAGS := -fdeclspec -Wall -Werror -fPIC -std=c99 -g
 else
 CC := gcc
+CFLAGS := -Wall -Werror -fPIC -std=c99 -g
 endif
 
-CFLAGS := -Wall -Werror -fPIC -std=c99 -g
 LDFLAGS :=
 
+ifeq ($(OS),Darwin)
+# Set the install_name of libtrufflerubytrampoline on macOS, so mkmf executables linking to it
+# will know they need to look at the rpath to find it.
+LIBS := -Wl,-install_name,@rpath/libtrufflerubytrampoline.$(SOEXT)
+else
+LIBS :=
+endif
+
 ROOT := $(realpath ../../../..)
 RUBY_HDR_DIR := $(ROOT)/lib/cext/include
 
 OBJECT_FILES := trampoline.o st.o strlcpy.o cext_constants.o
 
 libtrufflerubytrampoline.$(SOEXT): $(OBJECT_FILES) Makefile
-	$(Q) $(CC) -shared $(LDFLAGS) -o $@ $(OBJECT_FILES)
+	$(Q) $(CC) -shared $(LDFLAGS) -o $@ $(OBJECT_FILES) $(LIBS)
 
 %.o: %.c Makefile
 	$(Q) $(CC) -o $@ -c $(CFLAGS) $(LDFLAGS) -I$(RUBY_HDR_DIR) -I$(ROOT)/lib/cext/include/stubs -I$(TRUFFLE_NFI_NATIVE_INCLUDE) $<