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
causes the specified gcc to be used as the C compiler (unless it is overridden in the site shell script).
Unfortunately, this technique does not work for CONFIG_SHELL due to an Autoconf limitation. Until the limitation is lifted, you can use this workaround:
CONFIG_SHELL=/bin/bash ./configure CONFIG_SHELL=/bin/bash