build: Improve GCC version detection

This now makes sure that the beginning of the version number gets checked
and "4.4.5" will not match was a supported version.
GCC 8 and GCC 9 are now marked as supported, but we probably have to fix
some problems for them.

Closes: FS#1433
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

include/prereq-build.mk

@@ -28,13 +28,15 @@ $(eval $(call TestHostCommand,proper-umask, \
 
 $(eval $(call SetupHostCommand,gcc, \
 	Please install the GNU C Compiler (gcc) 4.8 or later \
-	$(CC) -dumpversion | grep -E '(4\.[8-9]|5\.?[0-9]?|6\.?[0-9]?|7\.?[0-9]?)', \
-	gcc -dumpversion | grep -E '(4\.[8-9]|5\.?[0-9]?|6\.?[0-9]?|7\.?[0-9]?)', \
+	$(CC) -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?)', \
+	gcc -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?)', \
 	gcc48 --version | grep gcc, \
 	gcc49 --version | grep gcc, \
 	gcc5 --version | grep gcc, \
 	gcc6 --version | grep gcc, \
 	gcc7 --version | grep gcc, \
+	gcc8 --version | grep gcc, \
+	gcc9 --version | grep gcc, \
 	gcc --version | grep Apple.LLVM ))
 
 $(eval $(call TestHostCommand,working-gcc, \
@@ -45,13 +47,15 @@ $(eval $(call TestHostCommand,working-gcc, \
 
 $(eval $(call SetupHostCommand,g++, \
 	Please install the GNU C++ Compiler (g++) 4.8 or later \
-	$(CXX) -dumpversion | grep -E '(4\.[8-9]|5\.?[0-9]?|6\.?[0-9]?|7\.?[0-9]?)', \
-	g++ -dumpversion | grep -E '(4\.[8-9]|5\.?[0-9]?|6\.?[0-9]?|7\.?[0-9]?)', \
+	$(CXX) -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?)', \
+	g++ -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?)', \
 	g++48 --version | grep g++, \
 	g++49 --version | grep g++, \
 	g++5 --version | grep g++, \
 	g++6 --version | grep g++, \
 	g++7 --version | grep g++, \
+	g++8 --version | grep g++, \
+	g++9 --version | grep g++, \
 	g++ --version | grep Apple.LLVM ))
 
 $(eval $(call TestHostCommand,working-g++, \