build: make GCC version 6+ minimal host build requirement

Currently minimal GNU supported GCC version is 7 (from May 2, 2017),
buildbots are using default GCC version 6 on Debian 9 (old stable),
current Debian stable has GCC version 8.3.0.

Signed-off-by: Petr Štetiar <[email protected]>

README.md

@@ -25,7 +25,7 @@ the [Build System Setup](https://openwrt.org/docs/guide-developer/build-system/i
 documentation.
 
 ```
-gcc binutils bzip2 flex python3 perl make4.1+ find grep diff unzip gawk getopt
+gcc-6+ binutils bzip2 flex python3 perl make4.1+ find grep diff unzip gawk getopt
 subversion libz-dev libc-dev rsync
 ```
 

include/prereq-build.mk

@@ -25,21 +25,21 @@ $(eval $(call TestHostCommand,proper-umask, \
 
 ifndef IB
 $(eval $(call SetupHostCommand,gcc, \
-	Please install the GNU C Compiler (gcc) 4.8 or later, \
-	$(CC) -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?|10\.?)', \
-	gcc -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?|10\.?)', \
+	Please install the GNU C Compiler (gcc) 6 or later, \
+	$(CC) -dumpversion | grep -E '^([6-9]\.?|10\.?)', \
+	gcc -dumpversion | grep -E '^([6-9]\.?|10\.?)', \
 	gcc --version | grep -E 'Apple.(LLVM|clang)' ))
 
 $(eval $(call TestHostCommand,working-gcc, \
-	\nPlease reinstall the GNU C Compiler (4.8 or later) - \
+	\nPlease reinstall the GNU C Compiler (6 or later) - \
 	it appears to be broken, \
 	echo 'int main(int argc, char **argv) { return 0; }' | \
 		gcc -x c -o $(TMP_DIR)/a.out -))
 
 $(eval $(call SetupHostCommand,g++, \
-	Please install the GNU C++ Compiler (g++) 4.8 or later, \
-	$(CXX) -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?|10\.?)', \
-	g++ -dumpversion | grep -E '^(4\.[8-9]|[5-9]\.?|10\.?)', \
+	Please install the GNU C++ Compiler (g++) 6 or later, \
+	$(CXX) -dumpversion | grep -E '^([6-9]\.?|10\.?)', \
+	g++ -dumpversion | grep -E '^([6-9]\.?|10\.?)', \
 	g++ --version | grep -E 'Apple.(LLVM|clang)' ))
 
 $(eval $(call TestHostCommand,working-g++, \