[skip ci] Add notes to CONTRIBUTING.md and sync variable names

Author: petk

CONTRIBUTING.md

@@ -235,8 +235,11 @@ locations.
     └─ cli/
        ├─ mime_type_map.h           # Generated by `sapi/cli/generate_mime_type_map.php`
        └─ ...
+    └─ embed/
+       ├─ php-embed.pc.in           # pkg-config metadata file
+       └─ ...
     └─ ...
- ├─ scripts/                        # php-config, phpize and internal development scripts
+ ├─ scripts/                        # php-config, phpize, pkg-config, and internal development scripts
  ├─ tests/                          # Core features tests
  └─ win32/                          # Windows build system files
     ├─ cp_enc_map.c                 # Generated by `win32/cp_enc_map_gen.exe`

sapi/embed/php-embed.pc.in

@@ -7,7 +7,7 @@ libdir=@orig_libdir@
 # PHP extension directory containing dynamically loadable PHP extensions.
 php_extension_dir=@EXTENSION_DIR@
 # PHP version as integer.
-php_version_id=@PHP_VERSION_ID@
+php_vernum=@PHP_VERSION_ID@
 # The path where to scan for additional INI configuration files.
 php_ini_dir=@EXPANDED_PHP_CONFIG_FILE_SCAN_DIR@
 # The path in which to look for php.ini.