build: Fix warning from inconsistency in gnu make

Make changed the interpretation of escaped # in a quote causing
warnings in the test for pthreads.

Change-Id: Ice94116713aea3c3e9725b38232e03f53d6633cc
Signed-off-by: Greg Tucker <[email protected]>

Author: gbtucker

make.inc

@@ -192,7 +192,7 @@ ASFLAGS += -DAS_FEATURE_LEVEL=$(as_feature_level) $(D_HAVE_AS_KNOWS_AVX512_y)
 
 
 # Check for pthreads
-have_threads ?= $(shell printf "\#include <pthread.h>\nint main(void){return 0;}\n" | $(CC) -x c - -o /dev/null -lpthread && echo y )
+have_threads ?= $(shell printf "int main(void){return 0;}\n" | $(CC) -x c - -o /dev/null -lpthread && echo y )
 THREAD_LD_$(have_threads) := -lpthread
 THREAD_CFLAGS_$(have_threads) := -DHAVE_THREADS