Fix handling of multi-defines on Windows

Author: Flamefire

ci/build.bat

@@ -19,7 +19,18 @@ SET B2_TOOLCXX=toolset=%B2_TOOLSET%
 
 IF DEFINED B2_CXXSTD (SET B2_CXXSTD=cxxstd=%B2_CXXSTD%)
 IF DEFINED B2_CXXFLAGS (SET B2_CXXFLAGS=cxxflags=%B2_CXXFLAGS%)
-IF DEFINED B2_DEFINES (SET B2_DEFINES=define=%B2_DEFINES%)
+IF DEFINED B2_DEFINES (
+    SETLOCAL EnableDelayedExpansion
+
+    SET "_B2_DEFINES="
+    FOR %%A IN (%B2_DEFINES%) DO (
+        SET "_B2_DEFINES=!_B2_DEFINES! define=%%~A"
+    )
+
+    SET "_B2_DEFINES=!_B2_DEFINES:~1!"
+
+    ENDLOCAL & SET "B2_DEFINES=%_B2_DEFINES%"
+)
 IF DEFINED B2_INCLUDE (SET B2_INCLUDE=include=%B2_INCLUDE%)
 IF DEFINED B2_ADDRESS_MODEL (SET B2_ADDRESS_MODEL=address-model=%B2_ADDRESS_MODEL%)
 IF DEFINED B2_TARGET_OS (SET B2_TARGET_OS=target-os=%B2_TARGET_OS%)