build: fix configuring with prebuilt libraries

Fix configure_library() to produce correct LDFLAGS when configuring with
prebuilt 3rd-party libraries (libuv, openssl, etc) using `pkg-config' or
`--shared-{LIBRARY}-includes=xxx --shared-{LIBRARY}-libpath=xxx'.

PR-URL: #3135
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

configure

@@ -749,11 +749,15 @@ def configure_library(lib, output):
     if pkg_cflags:
       output['include_dirs'] += (
           filter(None, map(str.strip, pkg_cflags.split('-I'))))
+    elif options.__dict__[shared_lib + '_includes']:
+      output['include_dirs'] += [options.__dict__[shared_lib + '_includes']]
 
     # libpath needs to be provided ahead libraries
     if pkg_libpath:
-      output['libraries'] += (
-          filter(None, map(str.strip, pkg_cflags.split('-L'))))
+      output['libraries'] += [pkg_libpath]
+    elif options.__dict__[shared_lib + '_libpath']:
+      output['libraries'] += [
+          '-L%s' % options.__dict__[shared_lib + '_libpath']]
 
     default_libs = getattr(options, shared_lib + '_libname')
     default_libs = map('-l{0}'.format, default_libs.split(','))