Build optimization for Python/SWIG interface

[oskooi]

The SWIG-generated Python interface is the slowest part of the Meep build. SWIG produces monolithic C++ files (typically 50K-100K+ lines) that are expensive to compile, and the current build configuration uses suboptimal SWIG flags and enforces unnecessary serial ordering across directories. The goal is to reduce wall-clock build time via changes to configure.ac and python/Makefile.am.

1. Reduce optimization level for SWIG-generated C++ compilation

The SWIG glue code is not performance-critical – hot loops live in libmeep.la. Compiling 50K+ lines at -O2/-O3 (set by AX_CXX_MAXOPT) wastes significant time on optimizations (vectorization, inlining) that don't benefit wrapper code.

2. Fix SWIG flags in both invocations

These flags primarily improve runtime call performance (10-30% for call-heavy code) but also modestly reduce generated .cxx file size, shaving a few second off compilation.

3. Reorder SUBDIRS to unblock parallel builds

File: Makefile.am (lines 3-18)

Currently: src -> tests -> [libpympb] -> python. The tests directory (17 C++ test programs) blocks libpympb from starting even though libpympb only depends on src/libmeep.la.

4. Add ccache support to configure.ac

Add an --enable-ccache option that prepends ccache to $CC and $CXX. Must go after CXX=$MPICXX assignment so it wraps the final compiler. Default to auto (use if found, skip silently otherwise).

For iterative development, this is the highest-impact single change: compiling the 50K+ line meep-python.cxx goes from 30-60s to ~1s on cache hit.

5. Suppress warnings on SWIG-generate code

SWIG-generated code produces many warnings that cannot be fixed. This is a developer-experience improvement rather than a build-time improvement.