Make FreeGEOS compile and run with more recent Open Watcom build

[mgroeber9110]

At the moment, README.md recommends using this build for compiling GEOS:

https://github.com/open-watcom/open-watcom-v2/releases/download/2020-12-01-Build/ow-snapshot.tar.gz

This is now almost 5 years old. However, switching to more recent builds has been found to trigger instabilities and crashes.

• For example, it has been reported that the 2024-04-03 snapshot of Open Watcom under 64-bit Linux leads to a crash in esp:

  Assertion failed: VM_IN_USE(block), function VMLock, vmLock.c, line 68
  

It may be worth trying to bisect snapshots between 2020 and 2024 to find out where problems were introduced (might be more than a single issue, because both the build tools and the target binaries could be broken), but hopefully we can find most of this with basic smoke testing to find a new stable snapshot to use.