Defer expansion of variable shared.ldflags

Fixes issue #64 ("shared.ldflags broken when building with
helper-library"), a regression introduced with commit
a6975e9 ("Reorder makefile sections"). Name of shared.lib was no
longer expanded after reordering sections.

Makefile.pdlibbuilder

@@ -493,7 +493,7 @@ ifeq ($(system), Linux)
   cxx.ldflags := -rdynamic -shared -fPIC -Wl,-rpath,"\$$ORIGIN",--enable-new-dtags
   cxx.ldlibs := -lc -lm -lstdc++
   shared.extension = so
-  shared.ldflags := -rdynamic -fPIC -shared -Wl,-soname,$(shared.lib)
+  shared.ldflags = -rdynamic -fPIC -shared -Wl,-soname,$(shared.lib)
 endif
 
 
@@ -648,7 +648,7 @@ depcheck.flags := $(cpp.flags) $(cflags)
 LDFLAGS := $(arch.ld.flags)
 
 # now add the same ld flags to shared dynamic lib
-shared.ldflags := $(shared.ldflags) $(LDFLAGS)
+shared.ldflags += $(LDFLAGS)
 
 # accumulated flags for C compiler / linker
 c.flags := $(cpp.flags) $(c.flags) $(cflags) $(CFLAGS)