Declare cfunction.fptr as jv (*)() to avoid having to cast everywhere

You only need to specify the return type in a function pointer
declaration in C.

If you use () in the declaration, the function pointer can be called
with any arguments, and the type of the arguments is decided for each
function call based on the types of the arguments used for the call.
(To declare a function pointer for a function with no arguments, you use
`(void)'.)

Since all the cfunction structs have a  fptr  that points to a functions
that return jv, not void, we can we can just declare cfunction.fptr as
jv (*)()  and avoid having those annoying and technically not C-standard
compliant casts everywhere.

Makefile.am

@@ -20,8 +20,7 @@ LIBJQ_SRC = src/builtin.c src/bytecode.c src/compile.c src/execute.c    \
 ### C build options
 
 AM_CFLAGS = -Wextra -Wall -Wno-missing-field-initializers               \
-	-Wno-unused-parameter -Wno-unused-function			\
-	-Wno-cast-function-type
+	-Wno-unused-parameter -Wno-unused-function
 
 if WIN32
 AM_CFLAGS += -municode