serdisplib: fix build on macos

./configure script detects if serdisplib is built on non-linux build
host and disables framebuffer driver. It blocks touchscreen_tool
compilation. This detection is not required on cross-compile build
so it is disabled via ac_cv_build=$(GNU_TARGET_NAME) in Makefile

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>

libs/serdisplib/Makefile

@@ -2,7 +2,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=serdisplib
 PKG_VERSION:=2.02
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=@SF/serdisplib
@@ -51,6 +51,9 @@ define Package/serdisplib-tools/description
  * touchscreen_tool
 endef
 
+CONFIGURE_VARS += \
+	ac_cv_build=$(GNU_TARGET_NAME)
+
 CONFIGURE_ARGS += \
 	--enable-dynloading \
 	--disable-statictools