meson: libpq: Revise static / shared library setup

Improvements:
- we don't need -DFRONTEND for libpq anymore since 1d77afefbd
- the .pc file contents for a static libpq were wrong (referencing
  {pgport, common}_shlib)
- incidentally fixes meson not supporting link_whole on AIX yet
- added explanatory comments

Previously I tried to avoid building libpq's sources twice, once for the
static and once for the shared library. We could still do so, but it's not
clear that it's worth the complication.

Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>

meson.build

@@ -2619,17 +2619,29 @@ backend_common_code = declare_dependency(
 
 subdir('src/common')
 
-frontend_shlib_code = declare_dependency(
-  compile_args: ['-DFRONTEND'],
-  include_directories: [postgres_inc],
+# all shared libraries should depend on shlib_code
+shlib_code = declare_dependency(
   link_args: ldflags_sl,
-  link_with: [pgport_shlib, common_shlib],
+)
+
+# all static libraries not part of the backend should depend on this
+frontend_stlib_code = declare_dependency(
+  include_directories: [postgres_inc],
+  link_with: [common_static, pgport_static],
   sources: generated_headers,
   dependencies: [os_deps, libintl],
 )
 
+# all shared libraries not part of the backend should depend on this
+frontend_shlib_code = declare_dependency(
+  include_directories: [postgres_inc],
+  link_with: [common_shlib, pgport_shlib],
+  sources: generated_headers,
+  dependencies: [shlib_code, os_deps, libintl],
+)
+
+# Dependencies both for static and shared libpq
 libpq_deps += [
-  frontend_shlib_code,
   thread_dep,
 
   gssapi,
@@ -2642,6 +2654,7 @@ subdir('src/interfaces/libpq')
 # fe_utils depends on libpq
 subdir('src/fe_utils')
 
+# for frontend binaries
 frontend_code = declare_dependency(
   include_directories: [postgres_inc],
   link_with: [fe_utils, common_static, pgport_static],