Trying to build and start modular Xorg release candidate 0, I get next message 
in logs:

Xorg: relocation error: /usr/local/lib/xorg/modules/drivers/mga_drv.so: 
undefined symbol: mga_read_and_process_bios

I tried to grep in xf86-video-mga-1.1.2 sources and found only this:

[mad@slowdown xf86-video-mga-1.1.2]$ grep -r mga_read_and_process_bios *
src/mga.h:Bool mga_read_and_process_bios(ScrnInfoPtr pScrn);
src/mga_driver.c:    mga_read_and_process_bios( pScrn );
src/mga_merge.c:    mga_read_and_process_bios( pScrn );

Seems that some file containing mga_read_and_process_bios is missing.