add SDL2 sources

devtools/makefile_base_posix.mak

@@ -134,13 +134,13 @@ else ifeq ($(USE_VALVE_BINDIR),1)
 	# Using /valve/bin directory.
 	export STEAM_RUNTIME_PATH ?= /valve
 	GCC_VER = -4.6
-	P4BIN = p4
+	P4BIN = $(SRCROOT)/devtools/bin/linux/p4
 	CRYPTOPPDIR=linux32
 else
 	# Not using chroot, use old steam-runtime. (gcc 4.6.3)
 	export STEAM_RUNTIME_PATH ?= /valve/steam-runtime
 	GCC_VER =
-	P4BIN = p4
+	P4BIN = true
 	CRYPTOPPDIR=ubuntu12_32
 endif