wasm: fix system lib detection and use

This is a revert of eea08d376a.
We need these flags to be added to the compiler in order to find the
emscripten ports to be able to use them.

Task-number: QTBUG-73127
Change-Id: Icf70f456947aef04dc79b2328f2e95fb1e94fcf8
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

mkspecs/wasm-emscripten/qmake.conf

@@ -21,6 +21,11 @@ EMCC_COMMON_LFLAGS = \
     --bind \
     -s \"BINARYEN_TRAP_MODE=\'clamp\'\"
 
+EMCC_USE_PORTS_FLAGS = \
+    -s USE_LIBPNG=1 \
+    -s USE_FREETYPE=1 \
+    -s USE_ZLIB=1
+
 # The -s arguments can also be used with release builds,
 # but are here in debug for clarity.
 EMCC_COMMON_LFLAGS_DEBUG = \
@@ -38,6 +43,9 @@ QMAKE_COMPILER         += emscripten
 QMAKE_CC                = emcc
 QMAKE_CXX               = em++
 
+QMAKE_CFLAGS           += $$EMCC_USE_PORTS_FLAGS
+QMAKE_CXXFLAGS         += $$EMCC_USE_PORTS_FLAGS
+
 # Practical debugging setup:
 #    "-g4" preserves function names for stack traces
 #    "-Os" produces reasonably sized binaries