configure & build: Rework legacy GCC detection to parse versions outside of Jam.

Jam comparison logic is string-based, and so was detecting GCC >= 10
as being < 2. This rectifies that by removing the GCC version parsing
from Jam logic entirely, and setting various BuildConfig variables
instead.

Change-Id: I0c0ae3b9002fb5e77f9ca7a78600c91871657f03
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3293
Reviewed-by: Adrien Destugues <[email protected]>
Reviewed-by: Jérôme Duval <[email protected]>
Reviewed-by: Alex von Gluck IV <[email protected]>

build/jam/BuildFeatureRules

@@ -447,7 +447,7 @@ rule InitArchitectureBuildFeatures architecture
 	# Add all secondary architectures as build features (with prefix).
 	EnableBuildFeatures secondary_$(TARGET_PACKAGING_ARCHS[2-]) ;
 
-	if $(TARGET_GCC_VERSION_$(architecture)[1]) = 2 {
+	if $(TARGET_CC_IS_LEGACY_GCC_$(architecture)) = 1 {
 		EnableBuildFeatures gcc2 ;
 	}