Error message: "AMDEP" was never defined

Question

I have a configure.ac in which I enable different compilers by different configure options. The configure script is generated successfully but at the end (call to AC_OUTPUT) I get an error message:

configure: error: conditional "AMDEP" was never defined.
Usually this means the macro was only invoked conditionally.

Sadly looking into config.log does not help and I have no idea what causes this error or where to start searching.

Answer 1

Apparently you cannot embed AM_PROG_CC, AM_PROG_CPP or AM_PROG_CXX in an if block (for whatever reason). Solution was to set a variable in the if block and pass it to AM_PROG_*.