String operations in Jamfiles

Question

How can I make a Jamfile path-independent ? As in

local headers = [ path.glob-tree /home/morpheus/base : *.hpp ] ;

install headers
    : $(headers)
    : <location>/home/morpheus/base_install <install-source-root>$(TOP)
    ;

project basetrade
    : requirements <include>/home/morpheus/base_install
      <variant>release:<cxxflags>-O2
    ;

to

local headers = [ path.glob-tree $(TOP) : *.hpp ] ;

install headers
    : $(headers)
    : <location>$(TOP)_install <install-source-root>$(TOP)
    ;

project basetrade
    : requirements <include>$(TOP)_install
      <variant>release:<cxxflags>-O2
    ;

Or perhaps define top level variables

alias SRCDIR /home/morpheus/base
alias DESTDIR /home/morpheus/base_install

Such that the user just needs to change these, and not the 15 odd Jamfiles in subdirectories ?