Config.in: add symbols for BR2_HOST_GCC_AT_LEAST_X_Y

Some host packages need a recent gcc version. Add symbols to Config.in
to specify the HOSTCC version. The values are passed through the
environment, and this environment is generated in a new support script.

Also update the documentation to mention the new symbols.

[Thomas: simplify by using only make logic instead of an external
shell script.]

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Config.in

@@ -18,6 +18,30 @@ config BR2_EXTERNAL
 	string
 	option env="BR2_EXTERNAL"
 
+# Hidden config symbols for packages to check system gcc version
+config BR2_HOST_GCC_VERSION
+	string
+	option env="HOST_GCC_VERSION"
+
+config BR2_HOST_GCC_AT_LEAST_4_7
+	bool
+	default y if BR2_HOST_GCC_VERSION = "4 7"
+
+config BR2_HOST_GCC_AT_LEAST_4_8
+	bool
+	default y if BR2_HOST_GCC_VERSION = "4 8"
+	select BR2_HOST_GCC_AT_LEAST_4_7
+
+config BR2_HOST_GCC_AT_LEAST_4_9
+	bool
+	default y if BR2_HOST_GCC_VERSION = "4 9"
+	select BR2_HOST_GCC_AT_LEAST_4_8
+
+config BR2_HOST_GCC_AT_LEAST_5
+	bool
+	default y if BR2_HOST_GCC_VERSION = "5"
+	select BR2_HOST_GCC_AT_LEAST_4_9
+
 # Hidden boolean selected by packages in need of Java in order to build
 # (example: xbmc)
 config BR2_NEEDS_HOST_JAVA