Add new make variable to seperate build library and target library

[worksofliam]

We need to have separate variables to allow gmake to look at objects in a one library but build in a different one. For example:

• BASE_LIB will default to BIN_LIB for backwards compatibility.
• this supports the standard git flow that is branch. You might have main which is library PRODAPP. This would be the BASE_LIB variable.
• Then when the user makes a new branch, they will use a new BIN_LIB but keep the BASE_LIB

BIN_LIB=VS12345
BASE_LIB=$(BIN_LIB)

/QSYS.LIB/$(BASE_LIB)/THEOBJ.PGM: qrpglesrc/theobj.pgm.rpgle
  system "CRTBNDRPG PGM($(BIN_LIB)) ..."

They have a .env like this:

BIN_LIB=&BRANCHLIB
BASE_LIB=&CURLIB