build clang8 only when $BUILD_CLANG8 is "true"

scripts/eosio_build_amazon.sh

@@ -149,7 +149,7 @@ if [ $? -ne 0 ]; then exit -1; fi
 
 printf "\\n"
 
-if [ $PIN_COMPILER ]; then
+if [ "$BUILD_CLANG8" = "true" ]; then
    printf "Checking Clang 8 support...\\n"
    if [ ! -d $CLANG8_ROOT ] || [ $FORCE_BUILD ]; then
       printf "Installing Clang 8...\\n"

scripts/eosio_build_centos.sh

@@ -209,7 +209,7 @@ printf "\\n"
 
 
 export CPATH="${CPATH}:${PYTHON3PATH}/root/usr/include/python3.6m" # m on the end causes problems with boost finding python3
-if [ $PIN_COMPILER ]; then
+if [ "$BUILD_CLANG8" = "true" ]; then
    printf "Checking Clang 8 support...\\n"
    if [ ! -d $CLANG8_ROOT ] || [ $FORCE_BUILD ]; then
       printf "Installing Clang 8...\\n"

scripts/eosio_build_darwin.sh

@@ -277,7 +277,7 @@ fi
 cd ..
 printf "\\n"
 
-if $PIN_COMPILER; then
+if [ "$BUILD_CLANG8" = "true" ]; then
    printf "Checking Clang 8 support...\\n"
    if [ ! -d $CLANG8_ROOT ] || [ $FORCE_BUILD ]; then
       printf "Installing Clang 8...\\n"

scripts/eosio_build_ubuntu.sh

@@ -162,7 +162,7 @@ if [ $? -ne 0 ]; then exit -1; fi
 
 printf "\\n"
 
-if [ $PIN_COMPILER ]; then
+if [ "$BUILD_CLANG8" = "true" ]; then
    printf "Checking Clang 8 support...\\n"
    if [ ! -d $CLANG8_ROOT ] || [ $FORCE_BUILD ]; then
       printf "Installing Clang 8...\\n"