gnuscripts/lalsuite_build.m4: fixes to LALSUITE_ADD_TESTS_ENV_CONFIG_VAR()

Description

• LALSUITE_ADD_TESTS_ENV_CONFIG_VAR():
  • remove duplicate values (e.g. if HAVE_PYTHON=true is already set in $TESTS_ENV_CONFIG_VARS, do not add HAVE_PYTHON=true again)
  • check for inconsistent values (e.g. raise an error if somehow the build system sets HAVE_PYTHON=true then HAVE_PYTHON=false)
• LALSUITE_CHECK_OPT_LIB():
  • remove LALSUITE_ENABLE_MODULE(), already called from LALSUITE_CHECK_LIB()
  • Turns out the extra LALSUITE_ENABLE_MODULE() call is needed after all, which is fine since the changes to LALSUITE_ADD_TESTS_ENV_CONFIG_VAR() will remove duplicate values

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

cc @adam-mercer