adjust compiler setup

- update to newer mpich
- find new compilers automatically, and display them
- force use of gcc 7.x

perf-regression/theta/packages.yaml

 packages:
+    all:
+        compiler: [gcc@7.3.0]
+        providers:
+            mpi: [mpich]
     mpich:
         modules: 
             mpich@7.7.3 arch=cray-cnl6-mic_knl: cray-mpich/7.7.3
@@ -53,6 +57,3 @@ packages:
         variants: fabrics=gni
     mercury:
         variants: +udreg
-    all:
-        providers:
-            mpi: [mpich]

perf-regression/theta/run-regression.sh

@@ -47,6 +47,8 @@ git checkout carns/dev-shell-detection
 patch -p1 < $SANDBOX/spack-libfabric-1.7.0.patch
 patch -p1 < $SANDBOX/spack-mercury-udreg-variant.patch
 . $SANDBOX/spack/share/spack/setup-env.sh
+spack find compilers
+spack compilers
 # put packages file in place in SPACK_ROOT to set our preferences for building
 # Mochi stack
 cp $SANDBOX/packages.yaml $SPACK_ROOT/etc/spack