Fixes and tweaks for configure.ac

[oskooi]

1: Upgrade minimum C++ standard from C++11 to C++17

Line 103: AX_CXX_COMPILE_STDCXX([11], [], [optiona])

C++11 is very old. The [optional] flag means compilation proceeds even without C++11 support. Given Meep is actively developed with modern C++ features, this should be at least C++14 or C++17 and [mandatory]. Note: the [mandatory] option indicates that C++17 is a minimum baseline rather than a strict version lock.

2: Duplicate AC_CHECK_LIB(m, ...) calls

Lines 150 and 233: AC_CHECK_LIB(m, sin) and AC_CHECK_LIB([m],[cos]) — libm is checked twice with different functions. The second check at line 233 is redundant since -lm is already in $LIBS from line 150. Remove one of them.

3: Inconsistent --with-openmp vs. --enable-openmp semantics

Line 139: Uses --with-openmp (via AC_ARG_WITH) but stores the result in enable_openmp=$enableval. The Autoconf convention is --enable-* for features and --with-* for external packages. OpenMP is a feature, so this should be AC_ARG_ENABLE(openmp, ...). The variable naming is also confusing — the argument is with_openmp but the assignment goes to enable_openmp, then line 140 checks $with_openmp:

Line 139: stores $enableval in enable_openmp
Line 140: but then checks $with_openmp

This works by accident (since $with_openmp is set by AC_ARG_WITH), but enable_openmp is never used.

4: MPICH SEEK_SET workaround is obsolete

Lines 77-99: The MPICH_IGNORE_CXX_SEEK workaround was for MPICH2 and very old MPI implementations. MPICH 3.x (released 2012) fixed this. Remove block and clean up src/fix_boundary_sources.cpp and src/mympi.cpp.

5: Obsolete Guile API checks

Lines 339-362: The checks for SCM_SMOB_PREDICATE, SCM_SMOB_DATA, SCM_NEWSMOB, and scm_make_smob_type test for APIs that have been standard since Guilde 1.8. These can be removed since Guile 2.0+ is a requirement.

6: Document BLAS/LAPACK variables

Added comment explaining $with_blas/$with_lapack are set by the AX_BLAS/AX_LAPACK m4 macros.

7: Add AC_CONFIG_AUX_DIR

Added AC_CONFIG_AUX_DIR([build-aux]), created build-aux/ directory; auxiliary files (install-sh, config.guess, etc.) live there.

8: Clarify FFTW detection

Added comment explaining FFTW2 fallback is intentional (code paths exist in monitor.cpp and casimir.cpp).

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.90%. Comparing base (f29a8c7) to head (632fbb9).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3179      +/-   ##
==========================================
+ Coverage   73.81%   73.90%   +0.09%     
==========================================
  Files          18       18              
  Lines        5423     5454      +31     
==========================================
+ Hits         4003     4031      +28     
- Misses       1420     1423       +3     

see 2 files with indirect coverage changes

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[stevengj]

I don't want to require this until we actually use C++17 features, which as far as I know we don't? I think the most recent feature we use is std::move, which is C++11. So we should just make C++11 mandatory.

[oskooi]

Fixed.

[stevengj]

I'm reluctant to rename this from --with-openmp to --enable-openmp, which could break a lot of people's build scripts for no good reason. In practice, configure scripts are not very consistent on with vs enable

[oskooi]

Reverted to --with-openmp.

[stevengj]

I think we can probably just delete FFTW2 support at this point.

[stevengj]

I'm a little reluctant to require guile 2; there are still a lot of guile 1.8 installations around.

[oskooi]

Reverted Guile 1.8 API checks.