LIBINT2_OPT_AM=0 breaks the build: #cmakedefine drops zero-valued LIBINT_*_AM macros #445

Symptom

Configuring with -DLIBINT2_OPT_AM=0 (scalar), or with a list whose maximum is 0 (e.g. -DLIBINT2_MAX_AM='2;1' -DLIBINT2_OPT_AM='0;0'), configures fine but the compiler fails to build:

src/bin/libint/build_libint.cc:601:34: error: use of undeclared identifier 'LIBINT_OPT_AM'

(line 601 on master, cparams->max_am_opt("default", LIBINT_OPT_AM);).

Cause

include/libint2/config.h.cmake.in declares the value with

#cmakedefine LIBINT_OPT_AM @LIBINT_OPT_AM@

#cmakedefine emits #define only when the CMake variable is true, and CMake treats 0 as false, so a zero value is written as /* #undef LIBINT_OPT_AM */. build_libint.cc then uses LIBINT_OPT_AM unconditionally (it is only #if defined-guarded for the _LIST variant).

The same #cmakedefine pattern is used for every LIBINT_<class>_MAX_AM and LIBINT_<class>_OPT_AM, so a user-specified 0 for any of them is silently dropped and the compiler falls back to the default (for MAX_AM this matters now that cmake/modules/int_am.cmake accepts 0, see #399). For OPT_AM, 0 is a legitimate request ("do not optimize/unroll anything") that currently cannot be expressed.

Reproduce

cmake -S . -B build -DLIBINT2_MAX_AM=1 -DLIBINT2_OPT_AM=0 -DLIBINT2_ENABLE_ERI=0
cmake --build build

Suggested fix

Emit the macros the way LIBINT_INCLUDE_<class> already is, so that 0 survives and only the unset sentinel is removed:

#define LIBINT_OPT_AM @LIBINT_OPT_AM@
#if @LIBINT_OPT_AM@ == -1
#undef LIBINT_OPT_AM
#endif

(with int_am.cmake setting the variable to -1 when it is not applicable), or guard the use in build_libint.cc with #if defined(LIBINT_OPT_AM). The first option also fixes the silent drop for the per-class _MAX_AM/_OPT_AM macros.

Found while verifying #399 with LIBINT2_ENABLE_GENERIC_CODE=ON (which needs a small opt AM to exercise the generic kernels).