src/make: use -c to compile instead of -r

The build currently warns about building an executable with an
object-file extension and recommends to use -c, -shared or -r
eventhough we already use -r.
I'm not sure what -r is supposed to do, but -c only compiles and not
links the input which should be exactly what we want here and using -c
gets rid of the warning without any apparent negative effects.

Author: TheOneric

src/Makefile

@@ -1,5 +1,6 @@
 # If needed, override the following variables via `make VARNAME=VAL ...`
-CXX = em++ -r
+# Note: we only compile and not(!) link here, thus -c
+CXX = em++ -c
 CXXFLAGS = -O3 -g4
 LIBASS_CXXFLAGS = $(shell pkg-config --static --cflags "libass >= 0.14.0")
 LOCAL_CXXFLAGS  = -Wall -Wno-deprecated $(LIBASS_CXXFLAGS)