configure.sh: quote local variables

fixes issue #711

specifying a multiword CC, e.g., CC='gcc -m32', would cause the failure
under dash

reported in
https://bugs.gentoo.org/show_bug.cgi?id=498136

patch by floppymaster at gmail dot com

Change-Id: I2ba246f765646161538622739961ec0f6c2d8c2d