How to link ruby pg to my own libpq.so

I have customized my own postgresql client and postgresql client-devel package and I want it to be available to Ruby programmer. So the command to install ruby pg gem is as below:

gem install pg  -- --with-pg-config=/usr/local/mypostgres-1.0/bin/pg_config --with-ldflags="-Wl,-rpath,/usr/local/mypostgres-1.0/lib"

The command works pretty good if there is only my own postgresql installed.

My pg_config output looks good:

BINDIR = /usr/local/mypostgres-1.0/bin 
DOCDIR = /usr/local/mypostgres-1.0/doc 
HTMLDIR = /usr/local/mypostgres-1.0/doc 
INCLUDEDIR = /usr/local/mypostgres-1.0/include 
PKGINCLUDEDIR = /usr/local/mypostgres-1.0/include 
INCLUDEDIR-SERVER = /usr/local/mypostgres-1.0/include/server 
LIBDIR = /usr/local/mypostgres-1.0/lib 
PKGLIBDIR开发者_StackOverflow = /usr/local/mypostgres-1.0/lib 
LOCALEDIR = /usr/local/mypostgres-1.0/share/locale 
MANDIR = /usr/local/mypostgres-1.0/man 
SHAREDIR = /usr/local/mypostgres-1.0/share 
SYSCONFDIR = /usr/local/mypostgres-1.0/etc
PGXS = /usr/local/mypostgres-1.0/lib/pgxs/src/makefiles/pgxs.mk 
VERSION = PostgreSQL 9.0.0

However, if there are both official and my customized postgresql installed. The installed driver will be linked against official libpq.so instead of my own libpq.so. Is there any way to force ruby-pg to be linked against my own libpq.so? Any clue or hint will be appreciated.

If I can't do this in this way, how would I customize the gem to link to my library?