See docs/user/clambc-user.pdf, chapter 1 "Installation".
Getting the bytecode compiler repository:
-----------------------------------------
git clone git://github.com/vrtadmin/clamav-bytecode-compiler

Quick start for building:
-------------------------
Check that your compiler version is not listed as a broken compiler here:
http://llvm.org/docs/GettingStarted.html#brokengcc

$ mkdir obj
$ cd obj
$ ../llvm/configure --enable-optimized --enable-targets=host-only --disable-bindings --prefix=/usr/local/clamav
$ make clambc-only -j4
$ make install-clambc -j8

For building and running tests:
-------------------------------
$ mkdir obj
$ cd obj
$ ../llvm/configure --enable-optimized --enable-targets=host-only --disable-bindings --prefix=/usr/local/clamav
$ make clambc-only -j4
$ make -j4
$ make check-all

See docs/user/clambc-user.pdf, section "1.3 Building" if build fails.

Staying up-to-date:
-------------------
git pull

Using a specific release:
-------------------------
$ git tag|grep ^clambc
clambc-0.10
$ git checkout clambc-0.10

Verifying signature on tag:
---------------------------
1. Get my GPG key (clamav.net/gpg/edwin.gpg)
2. Verify the tag:
$ git tag -v clambc-0.10

type commit
tag clambc-0.10
tagger Török Edvin <edwin@clamav.net> 1268418118 +0200

First public release of the ClamAV Bytecode Compiler.
gpg: Signature made Fri 12 Mar 2010 07:58:41 PM EET using RSA key ID 5379965D
gpg: Good signature from "Török Edvin <edwin@clamav.net>"
gpg:                 aka "Török Edvin <edwintorok@gmail.com>"