Use variables for selecting compilers

The latest and earliest clang/GCC compilers are now used through
variables instead of symlinks and also the all.sh script is updated
to support options for overriding the default values.

Signed-off-by: Gowtham Suresh Kumar <gowtham.sureshkumar@arm.com>
diff --git a/scripts/output_env.sh b/scripts/output_env.sh
index ce2c500..5356132 100755
--- a/scripts/output_env.sh
+++ b/scripts/output_env.sh
@@ -105,19 +105,35 @@
 print_version "gcc" "--version" "" "head -n 1"
 echo
 
-print_version "gcc-earliest" "--version" "" "head -n 1"
+if [ -n "${GCC_EARLIEST+set}" ]; then
+    print_version "${GCC_EARLIEST}" "--version" "" "head -n 1"
+else
+    echo " GCC_EARLIEST : Not configured."
+fi
 echo
 
-print_version "gcc-latest" "--version" "" "head -n 1"
+if [ -n "${GCC_LATEST+set}" ]; then
+    print_version "${GCC_LATEST}" "--version" "" "head -n 1"
+else
+    echo " GCC_LATEST : Not configured."
+fi
 echo
 
 print_version "clang" "--version" "" "head -n 2"
 echo
 
-print_version "clang-earliest" "--version" "" "head -n 2"
+if [ -n "${CLANG_EARLIEST+set}" ]; then
+    print_version "${CLANG_EARLIEST}" "--version" "" "head -n 2"
+else
+    echo " CLANG_EARLIEST : Not configured."
+fi
 echo
 
-print_version "clang-latest" "--version" "" "head -n 2"
+if [ -n "${CLANG_LATEST+set}" ]; then
+    print_version "${CLANG_LATEST}" "--version" "" "head -n 2"
+else
+    echo " CLANG_LATEST : Not configured."
+fi
 echo
 
 print_version "ldd" "--version" "" "head -n 1"