Make setting CHPL_DEVELOPER to a falsy value be respected

[jabraham17]

Makes sure setting CHPL_DEVELOPER to a falsy value is respected, by using the isTrue function in our Makefiles rather than ifdef.

Testing

• make clobber && make all frontend-shared && make check and then used file $(find build/runtime -name chpl-comm-diags.o); ls modules/TAGS; file $(find build -name ifa_vars.cpp.o); file $(find build -name codegen.cpp.o) to check if debug information was used
  • with CHPL_DEVELOPER unset
  • with CHPL_DEVELOPER=0 set
  • with CHPL_DEVELOPER=false set
  • with CHPL_DEVELOPER=1 set
  • with CHPL_DEVELOPER=true set
  • with CHPL_DEVELOPER=T set
  • with CHPL_DEVELOPER=n set
  • with CHPL_DEVELOPER=y set