[MacPorts] #55577: getting rid of unforeseen -I${prefix}/include option adding and its potential side-effects

[MacPorts]

#55577: getting rid of unforeseen -I${prefix}/include option adding and its
potential side-effects
-------------------------+-------------------------
 Reporter:  RJVB         |      Owner:
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  pcre, glib2
-------------------------+-------------------------
 I ran into header file conflicts the other day building code that depends
 indirectly on glib-2. This was on Linux using my MacPorts-for-Linux
 adaptation but the same thing could easily happen on Mac too because the
 same rules and guidelines apply for header search path priorities.

 The culprit was an unhappy -I/opt/local/include compiler option which
 caused a build to find headers from $prefix that were incompatible with
 the ones included and needed by the project.

 I traced that option first to  glib-2.0.pc which is patched by port:glib2
 to add the include path, and then to port:pcre which installs its headers
 into ${prefix}/include and thus actually needs it.

 The proper fix here is:

 1. Install the pcre (and pcre2) headers in a subdirectory with the
 official mechanism, i.e. add this to `configure.args` in `port file pcre`:
 {{{
                     --includedir=${prefix}/include/${subport} \
 }}}
 This change will be reflected in the pcre/pcre2 pkg-config files.

 2. update port:glib2 . This should be automatic but I didn't want to
 second-guess the reason NOT to use pkgconfig in `port configure glib2`
 which means a few changes are required. No hacks though, if anything the
 modified version is less of a hack than the original. `PCRE_CFLAGS` and
 `PCRE_LIBS` are now set by querying pkg-config instead of with hardcoded
 values (idem for the ZLIB variables, as a bonus). Assuming the change to
 glib-2.0.pc is still required, this can actually be done in such a way
 that the build system inserts the proper option(s) by adding
 `@PCRE_CFLAGS@` instead of the hardcoded `-I` option.

 After applying the changes above I get the pkg-config output below, which
 is far more specific
 {{{
 > pkg-config --cflags glib-2.0
 -I/opt/local/include/glib-2.0 -I/opt/local/lib/glib-2.0/include
 -I/opt/local/include/pcre
 }}}

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Ticket URL: <https://trac.macports.org/ticket/55577>
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