FindSleef: Use OpenMP in static builds

[fwcd]

Linking the vcpkg-built sleef statically on Linux seems to require OpenMP (see e.g. this build):

/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux/lib/libsleefdft.a(dftcommon.c.o): in function `SleefDFT_setPlanFilePath':
dftcommon.c:(.text+0x566): undefined reference to `GOMP_critical_start'
/usr/bin/ld: dftcommon.c:(.text+0x57c): undefined reference to `GOMP_critical_end'
/usr/bin/ld: dftcommon.c:(.text+0x68e): undefined reference to `omp_init_lock'
/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux/lib/libsleefdft.a(dftcommon.c.o): in function `PlanManager_loadMeasurementResultsP':
dftcommon.c:(.text+0x7d8): undefined reference to `GOMP_critical_start'
/usr/bin/ld: dftcommon.c:(.text+0x7fa): undefined reference to `GOMP_critical_end'
/usr/bin/ld: dftcommon.c:(.text+0x804): undefined reference to `omp_set_lock'
/usr/bin/ld: dftcommon.c:(.text+0xa03): undefined reference to `omp_unset_lock'
/usr/bin/ld: dftcommon.c:(.text+0xa39): undefined reference to `omp_unset_lock'
/usr/bin/ld: dftcommon.c:(.text+0xa66): undefined reference to `omp_init_lock'
/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux/lib/libsleefdft.a(dftcommon.c.o): in function `PlanManager_saveMeasurementResultsP':
dftcommon.c:(.text+0xab8): undefined reference to `GOMP_critical_start'
/usr/bin/ld: dftcommon.c:(.text+0xada): undefined reference to `GOMP_critical_end'
/usr/bin/ld: dftcommon.c:(.text+0xae4): undefined reference to `omp_set_lock'
/usr/bin/ld: dftcommon.c:(.text+0xdb6): undefined reference to `omp_init_lock'
...

[fwcd]

Looks like we may have to do this for libsleefdft rather than libsleef (note that there is no pkgconfig for sleefdft)...

[fwcd]

Related issue in upstream vcpkg: microsoft/vcpkg#32744

[fwcd]

Cleaning up this logic also has the side effect of making this dependency slightly more elegant.

[fwcd]

The build went through, so I will mark this one as ready for review too.

[daschuer]

LGTM, Thank you.