cmake: fix build with latest PETSc, SLEPc: lowercase pkg-config .pc files (patch included)

This PR fixes failure to find SLEPc due to a mismatch between slepc.pc installed in the system and SLEPc.pc that DOLFIN is looking for.

I do not see the FORK button, despite creating a new account (and using an old account). Cannot submit PR, so patch is aattached.

cmake: PETSc,SLEPc: match lowercase .pc pkg-config files

Upstream has moved to lowercase. PETSc has kept the mixed-case file for
compatibility, but SLEPc hasn't.

pkg_search_module takes multiple patterns and succeeds on first match,
so this commit is backward-compatible with older installations of PETSc,
SLEPc.

PETSC: https://gitlab.com/petsc/petsc/-/merge_requests/2754

commit 6fe8eb356ce6cf9027a2b889b1835f808039fe5e
Merge: fd0ee0fbd7 2eefe1c6f1
Author: Satish Balay <balay@mcs.anl.gov>
Date:   Tue Jun 9 01:08:49 2020 +0000

    Merge branch 'MR--config-lowercase-pkgname' into 'master'

    config: pkg-config: default to lowercase file.pc

    See merge request petsc/petsc!2754

SLEPc: https://gitlab.com/slepc/slepc/-/merge_requests/68

commit 1632350b0a681e0c4fd08c171912be6e17f3c3f7
Merge: 3c38a3419 b2fcc7199
Author: Jose E. Roman <jroman@dsic.upv.es>
Date:   Sun Jun 14 16:19:22 2020 +0000

    Merge branch 'jose/pkg-config' into 'master'

    pkg-config: default to lowercase file.pc

    See merge request slepc/slepc!68

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