build(header_install_dir-+-vendor): vendored quill and added header_install_dir option

this helps with wheel installs

BREAKING CHANGE:

meson.build

@@ -18,11 +18,18 @@
 #   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 #
 # *********************************************************************** #
-project('liblogging', 'cpp', version: 'v1.1.4', default_options: ['cpp_std=c++23'], meson_version: '>=1.5.0')
+project('liblogging', 'cpp', version: 'v1.1.5', default_options: ['cpp_std=c++23'], meson_version: '>=1.5.0')
 
 # Add default visibility for all C++ targets
 add_project_arguments('-fvisibility=default', language: 'cpp')
 
+logging_header_install_dir = get_option('header_install_dir') != '' \
+    ? get_option('header_install_dir') / 'fourdst' / 'logging'  \
+    : get_option('includedir') / 'fourdst' / 'logging'
+
+vendor_logging_header_install_dir = get_option('header_install_dir') != '' \
+    ? get_option('header_install_dir') / 'fourdst' / 'vendor'  \
+    : get_option('includedir') / 'fourdst' / 'vendor'
 
 subdir('build-config')
 subdir('src')
@@ -37,14 +44,15 @@ endif
 
 if get_option('pkg_config')
     message('Generating pkg-config file for liblogging...')
+    vendor_inc_flags = '-I' + vendor_logging_header_install_dir
     pkg = import('pkgconfig')
     pkg.generate(
         name: 'liblogging',
         description: 'Logging manager for SERiF and related projects',
         version: meson.project_version(),
         libraries: [liblogging,'-Wl,-rpath,${libdir}'],
-        subdirs: ['fourdst'],
         filebase: 'fourdst_logging',
-        install_dir: join_paths(get_option('libdir'), 'pkgconfig')
+        install_dir: join_paths(get_option('libdir'), 'pkgconfig'),
+        extra_cflags: vendor_inc_flags
     )
 endif