Use C style cast when converting void* into function pointer.

Use C style cast when converting void* into function pointer.

Building GiNaC 1.5.5 with GCC 3.4 fails with the following error:

libtool: compile:  ccache g++-3.4 -DHAVE_CONFIG_H -I. -I../../ginac -I../config -I/home/pc7135/varg/target/x86_64-linux-gnu/include -O2 -g -Wall -pipe -MT builtin_fcns.lo -MD -MP -MF .deps/builtin_fcns.Tpo -c ../../ginac/parser/builtin_fcns.cpp  -fPIC -DPIC -o .libs/builtin_fcns.o
../../ginac/parser/builtin_fcns.cpp: In function `GiNaC::ex (* GiNaC::encode_serial_as_reader_func(unsigned int))(const GiNaC::exvector&)':
/home/pc7135/varg/tmp/build/GiNaC/build-linux-gcc-3.4/ginac/../../ginac/parser/builtin_fcns.cpp|67| error: ISO C++ forbids casting between pointer-to-function and pointer-to-object
make[2]: *** [builtin_fcns.lo] Error 1

The C++98 standard [expr.reinterpret.cast] does not allow

reinterpret_cast<function_pointer>(void*)
reinterpret_cast<void*>(function_pointer)

But the ability to do so is important for a lot of practical uses. So soon
after the C++98 standard was approved, a language defect report was filed
on this topic, see
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#195
The result is that compilers will be allowed to support reinterpret_cast
conversions of function pointers to other (pointers) types, and vice a versa.
Such conversions work with *current* compilers (GCC 4.x), but don't work
with older ones, hence this patch.