Fix bug of src/Makefile.am & examples accept PREFIX variable. - src/Makefile.am install header files into $(prefix)/include/mb instead of $(prefix)/include original. - examples find headers, M4 macro files, and tools with PREFIX varaible. - default value of PREFIX is /usr/local. - user can change PREFIX with paramter of make command, for example 'make PREFIX=../../build/dest', if MadButterfly was installed there. - Users can install MadButterfly in their specified directory, for example, './configure --prefix=${PWD}/dest', the binary will installed in dest/.