add ability to set Free Pascal compiler binary path in build script

Author: zamronypj

build.cmd

@@ -18,5 +18,6 @@ IF NOT EXIST build.prod.cfg (copy build.prod.cfg.sample build.prod.cfg)
 IF NOT DEFINED BUILD_TYPE (SET BUILD_TYPE=prod)
 IF NOT DEFINED UNIT_OUTPUT_DIR (SET UNIT_OUTPUT_DIR=bin\unit)
 IF NOT DEFINED EXEC_OUTPUT_DIR (SET EXEC_OUTPUT_DIR=bin\out)
+IF NOT DEFINED FPC_BIN (SET FPC_BIN=fpc)
 
-fpc @unit.search.cfg @build.cfg src/fanocli.pas
+%FPC_BIN% @unit.search.cfg @build.cfg src/fanocli.pas