suppress warnings about incompatible casts with function pointers

Unfortunately we must rely on these in Purr Data. It's impossible to
enumerate every single function signature, especially given some are
dynamically loaded at runtime.

pd/src/makefile.in

@@ -41,7 +41,7 @@ LDFLAGS = @LDFLAGS@
 LIB =  @PDLIB@
 
 WARN_CFLAGS = -Wall -W -Wstrict-prototypes \
-    -Wno-unused-parameter -Wno-parentheses -Wno-switch
+    -Wno-unused-parameter -Wno-parentheses -Wno-switch -Wno-cast-function-type
 #WARN_CFLAGS += -Werror=implicit-function-declaration
 
 ARCH_CFLAGS = -DPD