Fix gcc version on Ubuntu 20.04 (#1030)

Author: DWJ01

scripts/installDeps.sh

@@ -82,6 +82,7 @@ then
   install_apt_deps
   if [[ "$OS_VERSION" =~ 20.04.* ]]
   then
+    install_gcc_7
     install_boost
   fi
 fi

scripts/installDepsUnattended.sh

@@ -68,6 +68,7 @@ then
     install_apt_deps
     if [[ "$OS_VERSION" =~ 20.04.* ]]
     then
+      install_gcc_7
       install_boost
     fi
   fi

scripts/installUbuntuDeps.sh

@@ -8,6 +8,14 @@ install_apt_deps(){
   ${SUDO} apt-get install m4 autoconf libtool automake cmake libfreetype6-dev libgstreamer-plugins-base1.0-dev -y
 }
 
+install_gcc_7(){
+  ${SUDO} apt-get install gcc-7 g++-7
+  ${SUDO} update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 100
+  ${SUDO} update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 100
+  ${SUDO} update-alternatives --set g++ /usr/bin/g++-7
+  ${SUDO} update-alternatives --set gcc /usr/bin/gcc-7
+}
+
 install_mediadeps_nonfree(){
   install_fdkaac
   install_ffmpeg