This workaround worked for me, but it is still hacky. Assuming the header file
is in the same directory than the nim file:
from os import splitPath
{.passC:"-I" & currentSourcePath().splitPath.head .}
Not tested, but the following could work.
const myHeader = currentSourcePath().splitPath.head & "/myHeader.h"
...
proc foo() {.importc, header: myHeader.}
