Add pkg-config support

Changes "make install" to add a package-config .pc file. Other programs can use this to get the correct cc and ld flags. e.g.

    $ pkg-config --cflags openlibm
    -I/usr/local/include 

    $ pkg-config --libs openlibm  
    -L/usr/local/lib -lopenlibm 

It also merges the `noinc` branch (https://github.com/JuliaLang/openlibm/pull/42) because otherwise the installed headers don't work at all.