[complib:mpfr] Fix building MPFR in some weird cases

The tmul test uses a compiled-in input file in $(srcdir).
The problem is that the Makefile passes it unquoted. The C code
tries to stringify it using clever macros, which may *usually* work.

In my case the source directory was named:
.../toolchain-powerpc-e500v2-linux-gnuspe-1.0-2.fc10/.../tests
And guess what? During testing I found out the program fails because
it tries to open:
.../toolchain-powerpc-e500v2-1-gnuspe-1.0-2.fc10/.../tests

Yes, CPP tokenized the macro before stringifying it and not surprisingly
the 'linux' part was converted to 1.
[on Fedora-10: cpp (GCC) 4.3.2 20081105 (Red Hat 4.3.2-7)]

So the attached patch simplify the macros and pass the path as string
from the Makefile.

 # PPL options

 choice

     bool

     prompt "PPL version"

 config PPL_V_0_10_2

     bool

     prompt "0.10.2"

 # CT_INSERT_VERSION_ABOVE

 # Don't remove above line!

 endchoice

 config PPL_VERSION

     string

     default "0.10.2" if PPL_V_0_10_2

 # CT_INSERT_VERSION_STRING_ABOVE

 # Don't remove above line!