Fix #254: libtool & distutils want certain compiler names.

This changes the compiler wrappers so that they are called by the same
name as the wrapped compiler.  Many builds make assumptions about
compiler names, and we need the spack compilers to be recognizable so
that build systems will get their flags right.

This adds per-compiler subdirectories to lib/spack/spack/env directory
that contain symlinks to cc for the C, C++, F77, and F90
compilers. The build now sets CC, CXX, F77, and F90 to point to these
links instead of to the generically named cc, c++, f77, and f90
wrappers.

lib/spack/env/xl/xlf90

@@ -0,0 +1 @@
+../cc