- GHA/windows/WinCE:
- set `-O3 -DNDEBUG` C flags manually for the CMake mingw32ce build.
CMake doesn't recognize the platform and fails to add them. To match
autotools (using `-O2`), and hit similar compiler warnings.
- enable parallel builds for cmake.
- tune parallelism for cmake using unity batches.
- tune parallelism for autotools.
Follow-up to 2a292c3984#15975
- tests: fix potentially uninitialized value in `readline()` in
`getpart.c`. Detected by gcc 4.4.0 `-O2` (Windows CE) jobs:
```
tests/server/getpart.c: In function 'getpart':
tests/server/getpart.c:298: error: 'datalen' may be used uninitialized in this function
```
Ref: https://github.com/curl/curl/actions/runs/13522595237/job/37785147505?pr=16476#step:11:25
Follow-up to 592880a3ca
- vtls_scache: rework returning pointer to avoid compiler warning seen
with `-O3` gcc 4.4.0 builds (Windows CE/schannel):
```
lib/vtls/schannel.c: In function 'schannel_connect_step1':
lib/vtls/vtls_scache.c:975: error: dereferencing pointer 'old_cred.4474' does break strict-aliasing rules
lib/vtls/vtls_scache.c:985: error: dereferencing pointer 'old_cred.4474' does break strict-aliasing rules
lib/vtls/schannel.c:959: note: initialized from here
```
Ref: https://github.com/curl/curl/actions/runs/13523868335/job/37789610845#step:9:25
Follow-up to fa0ccd9f1f#15774Closes#16476
c028a243f2