ci: Run headerscheck and cpluspluscheck in parallel

headerscheck ccache support

headerscheck: Use LLVM_CPPFLAGS

Remove no longer needed casts from Pointer

Remove no longer needed casts to Pointer

Change Pointer to void *

Don't rely on pointer arithmetic with Pointer type

Use more appropriate DatumGet* function

Further fix extended alignment for older g++.

Disable extended alignment uses on older g++

Work around buggy alignas in older g++

Use C11 alignas in pg_atomic_uint64 definitions

C11 alignas instead of unions -- extended alignments

Update comment related to C99

Fix pg_isblank()

Replace internal C function pg_hypot() by standard hypot()

Remove obsolete comment

Update timezone to C99

Remove obsolete cast