If your kernel fails to build, you probably have a perfectly explicable error. First, read the error message given by the compilation. Most of the time, the error message will explain what the kernel is missing. Generally, you will need to change your kernel configuration in some manner because of an error that config could not catch. A broken kernel compilation will end something like this:

  ../../../../arch/amd64/pci/pci_machdep.c: In function 1'pci_intr_map':
  ../../../../arch/amd64/pci/pci_machdep.c:641: error: 2'PCI_INT_VIA_ISA' undeclared (first use in this function)
  ../../../../arch/amd64/pci/pci_machdep.c:641: error: (Each undeclared identifier is reported only once
3 ../../../../arch/amd64/pci/pci_machdep.c:641: error: for each function it
  appears in.)
  *** Error code 1
4 Stop in /usr/src/sys/arch/amd64/compile/ENVY (line 89 of /usr/share/mk/sys.mk).

This kernel cannot be built because something is missing. When a build fails with statements that something “is undeclared” (as shown in bold), that’s a hint that the kernel is missing a necessary entry.

The name of where it failed might offer you a hint as to what’s missing. In this case, at 1, I have a function name where the compilation failed, and then a specific undeclared variable 2 that caused the compilation to fail.

I would start by figuring out where the pci_intr_map function comes from and what it’s supposed to do. Search the source code and man pages for references to the missing function. Failing that, try the mailing list archives. Be sure to include the function and variable names in any web search. Generic output that says that “there was an error” 3 or “the compile has stopped” 4 is less unique, and hence it could be useful. If all else fails, fall back to the GENERIC configuration.