Test each mono C# compiler to determine if it can handle generics.

The 'mcs' compiler on my system cannot handle generics.  This change to the
configure script macros tests each candidate mono C# compiler for generics
support.  If a candidate does not handle generics, then the next candidate
is tested.

m4/mercury.m4:
    As above.

    I have also stripped trailing whitespace the end of lines in this file.
Add support to allow the user to specify the C# compiler.

configure.ac:
    Add a new option, --with-csharp-compiler, that allows the user to
    specify what C# compiler to use.

m4/mercury.m4:
    If the user specified a C# compiler using the --with-csharp-compiler
    then use that.
Fix indentation in configure's help text.
Provide a helpful error if no C sharp compiler could be found

m4/mercury.m4:
    If the user specified one or more possible C sharp compilers and none
    of them were suitable, then report an error and abort.