Otherwise the build may pick up a different one which won't work since BEAM depends on its own modifications to pcre2.h.

Fixes:

beam/erl_bif_re.c:747:2: error: call to undeclared function 'pcre2_free_restart_data'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  747 |         pcre2_free_restart_data(rc->match_data);
      |         ^

(and several more) when building OTP-28.0 on a mac with pcre2.h installed in /opt/homebrew/include/.

The issue can occur on any system where CFLAGS has -I addons set to pick up additional headers not installed in the traditional locations known natively by the compiler, i.e. outside of /usr/include/. The end result is a compile command where those -I addons precede the -Ipcre that BEAM adds, and the wrong pcre2.h is selected.

Since BEAM has to use its own pcre2.h, the fix is to use an absolute path relative the source file when including it.