Log Message:
Use standard WRKSRC to allow patches against common/
Adapt Makefile variables for this.

While here, fix some build problems on NetBSD-6.99.15 with modular
xorg.

But:
Build on NetBSD-6.99.15/amd64 with modular xorg fails with:
gcc -O2 -ansi -Dasm=__asm GccWarningOptions   -I../../include \ 
-I../../exports/include/X11  -I../.. -I../../exports/include \ 
-I/usr/X11R6/include  -DCSRG_BASED  -DFUNCPROTO=15 -DNARROWPROTO     \ 
-DCPP_PROGRAM="\"cpp\"" -DHAS_MERGE_CONSTANTS=`if gcc \ 
-fmerge-constants -xc /dev/null -S -o /dev/null 2> /dev/null 1> /dev/null; \ 
then echo 1; else echo 0; fi`            -c imake.c
gcc: GccWarningOptions: No such file or directory
*** Error code 1