Node:Defining Variables, Next:configure Invocation, Previous:Sharing Defaults, Up:Running configure scripts
Variables not defined in a site shell script can be set in the
environment passed to configure. However, some packages may
run configure again during the build, and the customized values of these
variables may be lost. In order to avoid this problem, you should set
them in the configure command line, using VAR=value.
For example:
./configure CC=/usr/local2/bin/gcc
will cause the specified gcc to be used as the C compiler (unless it is overridden in the site shell script).