Add MacOS signing support with environment variable: CODESIGNIDENTITY. Remove the old code to attempt to sign it manually. Instead just specify the signing identity in CODESIGNIDENTITY. Most apps sign it in the "installer" rule, but since we don't have a DMG installer rule for the library, we instead sign it in the finishup script.

prefix=@prefix@
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${exec_prefix}/include
 
Name: dwindows
Description: Dynamic Windows Cross platform UI Toolkit
Version: @VERSION@
Libs: -L${prefix}/lib -ldwindows @LIBS@
Cflags: -I${prefix}/include @CFLAGS@ -D@DW_DEFINE@