Add pkg-config support

[talex5]

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 (#42) because otherwise the installed headers don't work at all.

[Keno]

Seems reasonable to me.