azure: avoid executing compiler if there is none

Until now, we always had the CC variable defined in the build.sh
pipeline. But as we're about to migrate the Windows jobs to Bash, as
well, those will not have CC defined and thus we cannot use "$CC" to
determine the compiler version.

Fix this by only executing "$CC" if the variable was set.

azure-pipelines/build.sh

@@ -10,7 +10,6 @@ set -e
 SOURCE_DIR=${SOURCE_DIR:-$( cd "$( dirname "${BASH_SOURCE[0]}" )" && dirname $( pwd ) )}
 BUILD_DIR=$(pwd)
 BUILD_PATH=${BUILD_PATH:=$PATH}
-CC=${CC:-cc}
 CMAKE=$(which cmake)
 
 indent() { sed "s/^/    /"; }
@@ -34,8 +33,11 @@ uname -a 2>&1 | indent
 
 echo "CMake version:"
 env PATH="$BUILD_PATH" "$CMAKE" --version 2>&1 | indent
-echo "Compiler version:"
-$CC --version 2>&1 | indent
+
+if test -n "$CC"; then
+	echo "Compiler version:"
+	"$CC" --version 2>&1 | indent
+fi
 echo ""
 
 echo "##############################################################################"