checker: add pkgconfig to comptime if (#10692)

doc/docs.md

@@ -4017,6 +4017,18 @@ If no flags are passed it will add `--cflags` and `--libs`, both lines below do
 The `.pc` files are looked up into a hardcoded list of default pkg-config paths, the user can add
 extra paths by using the `PKG_CONFIG_PATH` environment variable. Multiple modules can be passed.
 
+To check the existance of a pkg-config use `$pkgconfig('pkg')` as a compile time if condition to 
+check if a pkg-config exists. If it exists the branch will be created. Use `$else` or `$else $if`
+to handle other cases.
+
+```v ignore
+$if $pkgconfig('mysqlclient') {
+	#pkgconfig mysqlclient
+} $else $if $pkgconfig('mariadb') {
+	#pkgconfig mariadb
+}
+```
+
 ### Including C code
 
 You can also include C code directly in your V module.