fix passenger-install-nginx-module pcre2.h compile check

[sudoudaisuke]

Hello.

pcre2.h requires the definition of PCRE2_CODE_UNIT_WIDTH. The current code will always raise an error. The pcre_is_installed? method always returns false and always downloads pcre.tar.gz.

/usr/include/pcre2.h:971:2: error: #error PCRE2_CODE_UNIT_WIDTH must be defined before including pcre2.h.
  971 | #error PCRE2_CODE_UNIT_WIDTH must be defined before including pcre2.h.
      |  ^~~~~
/usr/include/pcre2.h:972:2: error: #error Use 8, 16, or 32; or 0 for a multi-width application.
  972 | #error Use 8, 16, or 32; or 0 for a multi-width application.
      |  ^~~~~

NGINX code sets PCRE2_CODE_UNIT_WIDTH to 8. https://github.com/nginx/nginx/blob/e734df6664e70f118ca3140bcef6d4f1750fa8fa/src/core/ngx_regex.h#L18

[sudoudaisuke]

@CamJN I fixed and signed it.