build: Adapt to backwards incompatible change in GNU Make 4.3
GNU Make 4.3 has a backwards incompatible change affecting the use of number signs or hashes (ie., #) inside function invocations. See: https://lists.gnu.org/archive/html/info-gnu/2020-01/msg00004.html
In this case, it would expand the '#' in the '\n#include "$(h)"' argument to the foreach call to '#', not '#'. This would lead to spurious backslashes in front of the '#include' directives in the generated fs-enumtypes.c file.
Spotted by Ernestas Kulik.