QT Source downloaded and used to build static Windows support, works.
mingw32-make -j8 clean after

emscriptem SDK is installed, sed in path

Configure line:
configure -no-warnings-are-errors -xplatform wasm-emscripten -platform win32-g++ -nomake examples -opensource -confirm-license -prefix %CD%\qtbase

Running configuration tests...
Checking for valid makespec... ERROR: Cannot compile a minimal program. The toolchain or QMakeSpec is broken.

Check config.log for details.

Command line: -no-warnings-are-errors -xplatform wasm-emscripten -platform win32-g++ -nomake examples -opensource -confirm-license -prefix C:\Qt\5.13.0\src\qtbase
executing config test verifyspec

• cd /d C:\Qt\5.13.0\src\config.tests\verifyspec && C:\Qt\5.13.0\src\qtbase\bin\qmake.exe "CONFIG -= qt debug_and_release app_bundle lib_bundle" "CONFIG += static warn_off console single_arch" -early "CONFIG += cross_compile" C:/Qt/5.13.0/src/qtbase/config.tests/verifyspec

Project MESSAGE: Setting TOTAL_MEMORY to 1GB
Project MESSAGE: Setting PTHREAD_POOL_SIZE to 4

• cd /d C:\Qt\5.13.0\src\config.tests\verifyspec && set MAKEFLAGS=& mingw32-make clean && set MAKEFLAGS=& mingw32-make

del verifyspec.obj
Could Not Find C:\Qt\5.13.0\src\config.tests\verifyspec\verifyspec.obj
del *~ core *.core
Could Not Find C:\Qt\5.13.0\src\config.tests\verifyspec*~
em++ -c -O3 -w -s USE_PTHREADS=1 -s TOTAL_MEMORY=1GB -s PTHREAD_POOL_SIZE=4 -IC:\Qt\5.13.0\src\qtbase\config.tests\verifyspec -I. -I/.emscripten_ports/openssl/include -IC:\Qt\5.13.0\src\qtbase\mkspecs\wasm-emscripten -o verifyspec.obj C:\Qt\5.13.0\src\qtbase\config.tests\verifyspec\verifyspec.cpp
process_begin: CreateProcess(NULL, em++ -c -O3 -w -s USE_PTHREADS=1 -s TOTAL_MEMORY=1GB -s PTHREAD_POOL_SIZE=4 -IC:\Qt\5.13.0\src\qtbase\config.tests\verifyspec -I. -I/.emscripten_ports/openssl/include -IC:\Qt\5.13.0\src\qtbase\mkspecs\wasm-emscripten -o verifyspec.obj C:\Qt\5.13.0\src\qtbase\config.tests\verifyspec\verifyspec.cpp, ...) failed.
make (e=2): Det går inte att hitta filen.
mingw32-make: *** [Makefile:1050: verifyspec.obj] Error 2

Any suggestions?