grep/pcre2: add GREP_PCRE2_DEBUG_MALLOC debug mode

grep/pcre2: add GREP_PCRE2_DEBUG_MALLOC debug mode

Add optional printing of PCREv2 allocations to stderr for a developer
who manually changes the GREP_PCRE2_DEBUG_MALLOC definition to "1".

You need to manually change the definition in the source file similar
to the DEBUG_MAILMAP, there's no Makefile knob for this.

This will be referenced a subsequent commit, and is generally useful
to manually see what's going on with PCREv2 allocations while working
on that code.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>