When configuring projects with the NVIDIA HPC SDK's C++ compiler
( https://developer.nvidia.com/hpc-sdk ) I find that the preprocessor
tends to add material from implicitly-included header files before the
main body of even a trivial program, which caused the regexp previously
here to fail to match, which caused a configure using boost.m4 to fail
at:

```
checking for Boost's header version...
configure: error: invalid value: boost_major_version=''
```