set clang to default compiler on ubuntu,amz2,macos

also bump the minimum version of clang the script looks for to 6.0 (cmake check is left at 5.0 for now still, still that still technically compiles eosio okay)

scripts/helpers/eosio.sh

@@ -121,8 +121,14 @@ function prompt-mongo-install() {
 }
 
 function ensure-compiler() {
-    export CXX=${CXX:-c++}
-    export CC=${CC:-cc}
+    DEFAULT_CXX=clang++
+    DEFAULT_CC=clang
+    if [[ $NAME == "CentOS Linux" ]]; then
+        DEFAULT_CXX=g++
+        DEFAULT_CC=gcc
+    fi
+    export CXX=${CXX:-$DEFAULT_CXX}
+    export CC=${CC:-$DEFAULT_CC}
     if $PIN_COMPILER || [[ -f $CLANG_ROOT/bin/clang++ ]]; then
         export PIN_COMPILER=true
         export BUILD_CLANG=true
@@ -141,9 +147,9 @@ function ensure-compiler() {
                 [[ $( $(which $CXX) --version | cut -d ' ' -f 4 | cut -d '.' -f 1 | head -n 1 ) -lt 10 ]] && export NO_CPP17=true
             else
                 if [[ $( $(which $CXX) --version | cut -d ' ' -f 3 | head -n 1 | cut -d '.' -f1) =~ ^[0-9]+$ ]]; then # Check if the version message cut returns an integer
-                    [[ $( $(which $CXX) --version | cut -d ' ' -f 3 | head -n 1 | cut -d '.' -f1) < 5 ]] && export NO_CPP17=true
+                    [[ $( $(which $CXX) --version | cut -d ' ' -f 3 | head -n 1 | cut -d '.' -f1) < 6 ]] && export NO_CPP17=true
                 elif [[ $(clang --version | cut -d ' ' -f 4 | head -n 1 | cut -d '.' -f1) =~ ^[0-9]+$ ]]; then # Check if the version message cut returns an integer
-                    [[ $( $(which $CXX) --version | cut -d ' ' -f 4 | cut -d '.' -f 1 | head -n 1 ) < 5 ]] && export NO_CPP17=true
+                    [[ $( $(which $CXX) --version | cut -d ' ' -f 4 | cut -d '.' -f 1 | head -n 1 ) < 6 ]] && export NO_CPP17=true
                 fi
             fi
         else