QMCPACK: compiler flags and mpi wrapper updates (#11326)

* Set mpi compiler wrappers using CC/etc. variables as recommended
  at https://spack.readthedocs.io/en/latest/packaging_guide.html#compiler-wrappers
  rather than setting CMake variables like -DCMAKE_C_COMPILER
* Use convenience functionality provided by inherited CMakePackage to
  export all compiler flags as CMake options

var/spack/repos/builtin/packages/qmcpack/package.py

@@ -169,6 +169,8 @@ class Qmcpack(CMakePackage, CudaPackage):
     patch_checksum = 'c066c79901a612cf8848135e0d544efb114534cca70b90bfccc8ed989d3d9dde'
     patch(patch_url, sha256=patch_checksum, when='@3.1.0:3.3.0')
 
+    flag_handler = CMakePackage.build_system_flags
+
     def patch(self):
         # FindLibxml2QMC.cmake doesn't check the environment by default
         # for libxml2, so we fix that.
@@ -180,16 +182,6 @@ def cmake_args(self):
         spec = self.spec
         args = []
 
-        # This bit of code is needed in order to pass compiler.yaml flags
-        # into the QMCPACK's CMake. Probably the CMake base class in
-        # the code of Spack should be doing this instead. Otherwise, it
-        # it would need to be done on a per package basis which is
-        # problematic.
-        cflags = spec.compiler_flags['cflags']
-        cxxflags = spec.compiler_flags['cxxflags']
-        args.append('-DCMAKE_C_FLAGS=%s' % ' '.join(cflags))
-        args.append('-DCMAKE_CXX_FLAGS=%s' % ' '.join(cxxflags))
-
         # This issue appears specifically with the the Intel compiler,
         # but may be an issue with other compilers as well. The final fix
         # probably needs to go into QMCPACK's CMake instead of in Spack.
@@ -200,12 +192,6 @@ def cmake_args(self):
         if '%intel' in spec:
             args.append('-DQMC_EXTRA_LIBS=-lifcore')
 
-        if '+mpi' in spec:
-            mpi = spec['mpi']
-            args.append('-DCMAKE_C_COMPILER={0}'.format(mpi.mpicc))
-            args.append('-DCMAKE_CXX_COMPILER={0}'.format(mpi.mpicxx))
-            args.append('-DMPI_BASE_DIR:PATH={0}'.format(mpi.prefix))
-
         # Currently FFTW_HOME and LIBXML2_HOME are used by CMake.
         # Any CMake warnings about other variables are benign.
         xml2_prefix = spec['libxml2'].prefix
@@ -318,10 +304,27 @@ def cmake_args(self):
     # QMCPACK 3.6.0 release and later has a functional 'make install',
     # the Spack 'def install' is retained for backwards compatiblity.
     # Note that the two install methods differ in their directory
-    # structure.
+    # structure. Additionally, we follow the recommendation on the Spack
+    # website for defining the compilers to be the MPI compiler wrappers.
+    # https://spack.readthedocs.io/en/latest/packaging_guide.html#compiler-wrappers
+    @when('@3.6.0:')
+    def install(self, spec, prefix):
+        if '+mpi' in spec:
+            env['CC'] = spec['mpi'].mpicc
+            env['CXX'] = spec['mpi'].mpicxx
+            env['F77'] = spec['mpi'].mpif77
+            env['FC'] = spec['mpi'].mpifc
+
+        with working_dir(self.build_directory):
+            make('install')
+
     @when('@:3.5.0')
     def install(self, spec, prefix):
-        """Make the install targets"""
+        if '+mpi' in spec:
+            env['CC'] = spec['mpi'].mpicc
+            env['CXX'] = spec['mpi'].mpicxx
+            env['F77'] = spec['mpi'].mpif77
+            env['FC'] = spec['mpi'].mpifc
 
         # QMCPACK 'make install' does nothing, which causes
         # Spack to throw an error.