mkmf.rb: Define missing POSTLINK variable in generated Makefile

The `POSTLINK` variable had been used in the `LINK_SO` variable, which
is used to link shared extension libraries. However, the `POSTLINK`
variable had not been defined in the generated Makefile, so extension
libraries were not properly post-processed. It was not a critical issue
for the existing `POSTLINK` usage for darwin platforms, but it would be
a problem for Wasm/WASI platform, which requires *mandatory*
post-processing for shared extension libraries.

lib/mkmf.rb

@@ -2148,6 +2148,7 @@ ARCH_FLAG = #{$ARCH_FLAG}
 DLDFLAGS = $(ldflags) $(dldflags) $(ARCH_FLAG)
 LDSHARED = #{CONFIG['LDSHARED']}
 LDSHAREDXX = #{config_string('LDSHAREDXX') || '$(LDSHARED)'}
+POSTLINK = #{config_string('POSTLINK', RbConfig::CONFIG)}
 AR = #{CONFIG['AR']}
 LD = #{CONFIG['LD']}
 EXEEXT = #{CONFIG['EXEEXT']}