how to redefine variable in makefile

[Joylene Nettleton]

I need to generate two files using one source, but with two different
version of Oracle pre-processors in a makefile.  The Oracle pre-processor
requires to set ORACLE_HOME variable to point to a specific Oracle version.
How could I do this?  I tried the following way, but won't work:

output_a : input_main
        export ORACLE_HOME=oracle-version-a
        <pre-process command>

output_b : input_main
        export ORACLE_HOME=oracle-version-b
        <pre-process command> 

Thanks much in advance for your help.

--Joylene