LOCDIR: fix all outdated paths and normalize trailing slash

Update performed by:

git ls-files '*/makefile' | xargs perl -pi -MFile::Basename \
-e 's!(LOCDIR[[:space:]]+=) *(.+)$!$1." ".dirnam

LOCDIR: fix all outdated paths and normalize trailing slash

Update performed by:

git ls-files '*/makefile' | xargs perl -pi -MFile::Basename \
-e 's!(LOCDIR[[:space:]]+=) *(.+)$!$1." ".dirname($ARGV)."/"!e'

Fortran auto must be fixed using 'make allfortranstubs' and the result
can be checked by 'bin/maint/checkloc.py'.

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Change all PETSC_EXTERN_C to PETSC_EXTERN

Hg-commit: 8d2ebbb193fb583bccc64015e35640c4e08c3426

began removing unneeded EXTERN_C_BEGIN/END and converting needed ones to PETSC_EXTERN_C

Hg-commit: e0c157606a3283d5a4ba8902a0da5cad0e2aa531

added PetscObjectComm() and used it to replace (((PetscObject)obj)->comm)

Hg-commit: 3da37c458124ad48ae939f4e9823e4430ee0b8be

Adjusted check for NULL instead of PETSC_NULL, fixed remaining uses of PETSC_NULL

Hg-commit: 6aa0365770ae203a0249581a0e63fa289e95b4dd