ppc64le: Build fails with `pngrutil.c:(.text+0x6658): undefined reference to `png_init_filter_functions_vsx'`

Dear LibreOffice folks,


On ppc64le with Ubuntu 20.04 and latest LibreOffice from master, the 
build fails with the error below.

```
$ uname -m
ppc64le
$ git describe --dirty
libreoffice-7-0-branch-point-1865-g69a33480fe59
$ git log --oneline -1
69a33480fe59 (HEAD -> master) sw: silence 'not all control paths return 
a value'
$ ./autogen.sh
$ make -j100
[…]
[LNK] Executable/bestreversemap
[DEP] LNK:Executable/oosplash
[LNK] Executable/oosplash
[DEP] LNK:Executable/gendict
[LNK] Executable/gendict
[DEP] LNK:Executable/genconv_dict
[LNK] Executable/genconv_dict
[DEP] LNK:Library/libjpipe.so
[LNK] Library/libjpipe.so
[DEP] LNK:Executable/opencltest
[LNK] Executable/opencltest
[DEP] LNK:Executable/osl_process_child
[LNK] Executable/osl_process_child
[PAT] nss
[BIN] salhelper
/usr/bin/ld: 
/home/pmenzel/src/core/workdir/LinkTarget/StaticLibrary/liblibpng.a(pngrutil.o): 
in function `png_read_filter_row':
pngrutil.c:(.text+0x6658): undefined reference to 
`png_init_filter_functions_vsx'
collect2: error: ld returned 1 exit status
make[1]: *** [/home/pmenzel/src/core/desktop/Executable_oosplash.mk:10: 
/home/pmenzel/src/core/instdir/program/oosplash] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:282: build] Error 2
```


Kind regards,

Paul