-DEMSCRIPTEN=1 appears to not be present any more by default in emcc compiler.
There is -D__EMSCRIPTEN__=1 but that would require modifying many locations
in the codebase which are already being reviewed separately under a single
source multi-platform build cleanup process.
Anyhow maybe ./configure would set -DEMSCRIPTEN=1 itself, much like -Dlinux=1
for other targets. This commits defers evaluation on that until later and just
restores how it worked before.
Darryl L. Miles