Add a dummy file, to make sure the library is non-empty

Should fix the macOS build.

Makefile.am

@@ -8,7 +8,7 @@ noinst_LTLIBRARIES = librsvg_c_api.la
 # This is empty, because automake wants to have a C sources list for
 # the LTLIBRARIES.  See LIBRSVG_INTERNALS_SRC below for the real list
 # of Rust sources.
-librsvg_c_api_la_SOURCES =
+librsvg_c_api_la_SOURCES = librsvg/dummy.c
 
 bin_PROGRAMS = rsvg-convert
 
@@ -182,7 +182,7 @@ dist-hook:
 	mkdir .cargo &&								\
 	cp cargo-vendor-config .cargo/config)
 
-librsvg_c_api.la: $(LIBRSVG_INTERNALS_SRC) $(LIBRSVG_C_API_SRC)
+librsvg_c_api.la: $(librsvg_c_api_la_OBJECTS) $(LIBRSVG_INTERNALS_SRC) $(LIBRSVG_C_API_SRC)
 	+cd $(top_srcdir)/librsvg &&						\
 	PKG_CONFIG_ALLOW_CROSS=1						\
 	PKG_CONFIG='$(PKG_CONFIG)'						\
@@ -191,7 +191,7 @@ librsvg_c_api.la: $(LIBRSVG_INTERNALS_SRC) $(LIBRSVG_C_API_SRC)
 	&& if [[ $$($(NM) -g $(RUST_LIB) | grep "T __*clzsi2" -c) -gt 1 ]] ; then \
 		$(AR) d $(RUST_LIB) clzsi2.o;					\
 	fi \
-	&& cd $(LIBRSVG_BUILD_DIR) && $(LINK) && mv $(RUST_LIB) .libs/librsvg_c_api.a
+	&& cd $(LIBRSVG_BUILD_DIR) && $(LINK) $< && mv $(RUST_LIB) .libs/librsvg_c_api.a
 
 librsvg_@RSVG_API_MAJOR_VERSION@_la_CPPFLAGS = \
 	-I$(top_srcdir) 			\

librsvg/dummy.c

@@ -0,0 +1,30 @@
+/* A dummy file, to prevent empty libraries from breaking builds.
+
+   This file is in the public domain.  -- Bruno Haible */
+
+/* Some systems, reportedly OpenBSD and Mac OS X, refuse to create
+   libraries without any object files.  You might get an error like:
+
+   > ar cru .libs/libgl.a
+   > ar: no archive members specified
+
+   Compiling this file, and adding its object file to the library, will
+   prevent the library from being empty.  */
+
+/* Some systems, such as Solaris with cc 5.0, refuse to work with libraries
+   that don't export any symbol.  You might get an error like:
+
+   > cc ... libgnu.a
+   > ild: (bad file) garbled symbol table in archive ../gllib/libgnu.a
+
+   Compiling this file, and adding its object file to the library, will
+   prevent the library from exporting no symbols.  */
+
+#ifdef __sun
+/* This declaration ensures that the library will export at least 1 symbol.  */
+int gl_dummy_symbol;
+#else
+/* This declaration is solely to ensure that after preprocessing
+   this file is never empty.  */
+typedef int dummy;
+#endif