Fix #187: Add new variable CFLAGS_FOR_BUILD for CC_FOR_BUILD compiles.

Fix #187: Add new variable CFLAGS_FOR_BUILD for CC_FOR_BUILD compiles.

Due to the need to support cross-building, CC_FOR_BUILD is used instead of
CC for tools that will be run as part of the build process.  However, it
is sometimes necessary to add custom CFLAGS for these builds; CFLAGS_FOR_BUILD
supports this cleanly.

svn path=/trunk/yasm/; revision=2234