Use native variable validation

* Use null utility `: ...` to avoid execution
* Use `${parameter:?[word]}` to do ensure the variable is set
* Drop `eval` loop

The error message on Ubuntu reads:

> cc.sh: 205: SPACK_COMPILER_WRAPPER_PATH: Error: compiler wrapper must be invoked by Spack

Previously it was:

> [spack cc]: Error: Spack compiler must be run from Spack! Input 'SPACK_COMPILER_WRAPPER_PATH' is missing.

Signed-off-by: Harmen Stoppels <me@harmenstoppels.nl>

Author: haampie

cc.sh

@@ -32,17 +32,6 @@ unset IFS
 # NOTE: Depending on your editor this may look empty, but it is not.
 readonly lsep=''
 
-# This is an array of environment variables that need to be set before
-# the script runs. They are set by routines in spack.build_environment
-# as part of the package installation process.
-readonly params="\
-SPACK_COMPILER_WRAPPER_PATH
-SPACK_DEBUG_LOG_DIR
-SPACK_DEBUG_LOG_ID
-SPACK_SHORT_SPEC
-SPACK_SYSTEM_DIRS
-SPACK_MANAGED_DIRS"
-
 # Optional parameters that aren't required to be set
 
 # Boolean (true/false/custom) if we want to add debug flags
@@ -212,12 +201,13 @@ if eval "[ \"\${*#*${lsep}}\" != \"\$*\" ]"; then
     die "Compiler command line contains our separator ('${lsep}'). Cannot parse."
 fi
 
-# ensure required variables are set
-for param in $params; do
-    if eval "test -z \"\${${param}:-}\""; then
-        die "Spack compiler must be run from Spack! Input '$param' is missing."
-    fi
-done
+# ensure required variables are set (POSIX 2.6.2 ${parameter:?word})
+: "${SPACK_COMPILER_WRAPPER_PATH:?Error: compiler wrapper must be invoked by Spack}"
+: "${SPACK_DEBUG_LOG_DIR:?Error: compiler wrapper must be invoked by Spack}"
+: "${SPACK_DEBUG_LOG_ID:?Error: compiler wrapper must be invoked by Spack}"
+: "${SPACK_SHORT_SPEC:?Error: compiler wrapper must be invoked by Spack}"
+: "${SPACK_SYSTEM_DIRS:?Error: compiler wrapper must be invoked by Spack}"
+: "${SPACK_MANAGED_DIRS:?Error: compiler wrapper must be invoked by Spack}"
 
 # eval this because SPACK_MANAGED_DIRS and SPACK_SYSTEM_DIRS are inputs we don't wanna loop over.
 # moving the eval inside the function would eval it every call.