In a lot of Linux kernel modules (hardware drivers) their source header files, one can spot lines like:
#ifndef __VMKLNX__
__VMKLNX__ is not defined in the source code of this module. A guess is that __VMKLNX__ is a "C" system specific predefined macro. Maybe related to virtual machines or even VMware. However I can't find any authoritative source to confirm this.
What is and how/where can I find the meaning of this __VMKLNX__ part?
That means the driver supports being built for the VMware ESX kernel, whose documentation and source code are not public. The VMware ESX kernel is a hypervisor commonly called "vmkernel". You can infer what __VMKLNX__ is for by looking at code comments in existing drivers, like in bnx2.c:
#if defined(__VMKLNX__)
/* On VMware ESX there is a possibility that that netdev watchdog thread
* runs before the reset task if the machine is loaded. If this occurs
* too many times, these premature watchdog triggers will cause a PSOD
* on a VMware ESX beta build */
#define TX_TIMEOUT (20*HZ)
#else
#define TX_TIMEOUT (5*HZ)
#endif /* defined(__VMKLNX__) */
VMware ESX isn't linux, though. The ESX kernel just implements an interface that allows it to run Linux device drivers.