Allow user to disable GiNaC::compile_ex (e.g. for security reasons).

configure takes --{disable,enable}-excompiler argument now. It can be
used to disable GiNaC::compile_ex (default is to enable it).

acinclude.m4:
GINAC_EXCOMPILER: new macro. Checks for libdl, allows user to disable
GiNaC::compile_ex. Also it doesn't bother to check for libdl on MinGW.

configure.ac: use GINAC_EXCOMPILER to check for libdl.