Change: [Build] use flags set by env at the end, not at the start (ba…

…sed on patch by adf88) (OpenTTD#6715)

This closes OpenTTD#6433

config.lib

@@ -1481,21 +1481,27 @@ make_compiler_cflags() {
 
 make_cflags_and_ldflags() {
 	# General CFlags for BUILD
-	CFLAGS_BUILD="$CFLAGS_BUILD"
+	CFLAGS_BUILD_ENV="$CFLAGS_BUILD"
+	CFLAGS_BUILD=""
 	# Special CXXFlags for BUILD
-	CXXFLAGS_BUILD="$CXXFLAGS_BUILD"
+	CXXFLAGS_BUILD_ENV="$CXXFLAGS_BUILD"
+	CXXFLAGS_BUILD=""
 	# LDFLAGS for BUILD
-	LDFLAGS_BUILD="$LDFLAGS_BUILD"
+	LDFLAGS_BUILD_ENV="$LDFLAGS_BUILD"
+	LDFLAGS_BUILD=""
 	# FEATURES for BUILD (lto)
 	FEATURES_BUILD=""
 	# General CFlags for HOST
-	CFLAGS="$CFLAGS"
+	CFLAGS_ENV="$CFLAGS"
+	CFLAGS=""
 	# Special CXXFlags for HOST
-	CXXFLAGS="$CXXFLAGS"
+	CXXFLAGS_ENV="$CXXFLAGS"
+	CXXFLAGS=""
 	# Libs to compile. In fact this is just LDFLAGS
 	LIBS="-lstdc++"
 	# LDFLAGS used for HOST
-	LDFLAGS="$LDFLAGS"
+	LDFLAGS_ENV="$LDFLAGS"
+	LDFLAGS=""
 	# FEATURES for HOST (lto)
 	FEATURES=""
 
@@ -1913,6 +1919,14 @@ make_cflags_and_ldflags() {
 		fi
 	fi
 
+	# All flags to be extended via the env
+	CFLAGS_BUILD="$CFLAGS_BUILD $CFLAGS_BUILD_ENV"
+	CXXFLAGS_BUILD="$CXXFLAGS_BUILD $CXXFLAGS_BUILD_ENV"
+	LDFLAGS_BUILD="$LDFLAGS_BUILD $LDFLAGS_BUILD_ENV"
+	CFLAGS="$CFLAGS $CFLAGS_ENV"
+	CXXFLAGS="$CXXFLAGS $CXXFLAGS_ENV"
+	LDFLAGS="$LDFLAGS $LDFLAGS_ENV"
+
 	log 1 "using CFLAGS_BUILD... $CFLAGS_BUILD"
 	log 1 "using CXXFLAGS_BUILD... $CXXFLAGS_BUILD"
 	log 1 "using LDFLAGS_BUILD... $LDFLAGS_BUILD"