Out-of-tree builds fail

Building mpiP outside of teh source tree fails for me.

Reproducing the problem is simple:

```
cd mpiP-3.4.1
mkdir _build; cd __build
../configure ....
make shared
make install
```

I run into two issues:
1. The header file `mpiPi.h` is not found during compilation.
A quick fix is to have `CFLAGS="-I.",` but the build system should deal with this.
2. Later on, I get the error:
```
/usr/bin/install -c doc/*txt doc/*html doc/README ...../share/doc/mpip ; \

/usr/bin/install: cannot stat ‘doc/*txt’: No such file or directory
/usr/bin/install: cannot stat ‘doc/*html’: No such file or directory
/usr/bin/install: cannot stat ‘doc/README’: No such file or directory
```

Reproduce the out-of-tree build from the issue using configure, make shared, and make install. Inspect the build rules involving mpiPi.h and the doc/*txt, doc/*html, and doc/README install paths. Done means an out-of-tree build completes compilation and installation without missing-header or missing-document errors.

Written by the indexing model from the issue text.