Fix libjpeg detection (stray backslash when using /bin/echo)

src/Makefile

@@ -16,7 +16,7 @@ ifeq ($(shell PKG_CONFIG_PATH=$(PKG_CONFIG_PATH) pkg-config --exists libpng || e
   $(error "libpng is required - install libpng-dev")
 endif
 
-ifeq ($(shell $(ECHO) "\#include <stdio.h>\n\#include <jpeglib.h>\nint main() { struct jpeg_error_mgr jerr; jpeg_std_error(&jerr); return 0; }" | $(CC) -pipe -x c -o $(CONFIGTEMP) $(ARCHINCLUDES) $(LDFLAGS) - -ljpeg 2>/dev/null || echo no; $(RM) -f $(CONFIGTEMP)), no)
+ifeq ($(shell $(ECHO) "#include <stdio.h>\n#include <jpeglib.h>\nint main() { struct jpeg_error_mgr jerr; jpeg_std_error(&jerr); return 0; }" | $(CC) -pipe -x c -o $(CONFIGTEMP) $(ARCHINCLUDES) $(LDFLAGS) - -ljpeg 2>/dev/null || echo no; $(RM) -f $(CONFIGTEMP)), no)
   $(error "libjpeg is required - install libjpeg-dev, libjpeg8-dev or libjpeg62-dev")
 endif