Fix typos in comments.
Consistently define BUILDING_DLL during builds of src/port for Windows.

The MSVC build process already did so; this fixes the principal build
process to match.  Both processes already did likewise for src/common.
This lets server builds of src/port reference postgres.exe data symbols.
Have configuration templates augment, not replace, LDFLAGS.

This preserves user-specified LDFLAGS; we already kept user-specified
CFLAGS and CPPFLAGS.  Given the shortage of complaints and the fact that
any problem caused is likely to appear at build time, no back-patch.

Dag-Erling Smørgrav and Noah Misch