MPI detection: mpicc/mpiicc before pkg-config

The standard way to compile MPI applications (recommanded by all MPI implementations)
is to use the commands mpicc/mpiicc (and friends). Therefore, it is standard to
just set PATH such that mpicc points towards a wrapper of the MPI implementation
that one wants to use.

In contrast, pkg-config is supported only by OpenMPI.

Therefore, Meson has first to take into account the mpicc command to get chance
to use mpicc of MPICH or IntelMPI in the case OpenMPI is installed
(so that pkg-config would find it).

mesonbuild/dependencies/mpi.py

@@ -37,18 +37,6 @@ def mpi_factory(env: 'Environment',
         return []
     compiler_is_intel = compiler.get_id() in {'intel', 'intel-cl'}
 
-    # Only OpenMPI has pkg-config, and it doesn't work with the intel compilers
-    if DependencyMethods.PKGCONFIG in methods and not compiler_is_intel:
-        pkg_name = None
-        if language == 'c':
-            pkg_name = 'ompi-c'
-        elif language == 'cpp':
-            pkg_name = 'ompi-cxx'
-        elif language == 'fortran':
-            pkg_name = 'ompi-fort'
-        candidates.append(functools.partial(
-            PkgConfigDependency, pkg_name, env, kwargs, language=language))
-
     if DependencyMethods.CONFIG_TOOL in methods:
         nwargs = kwargs.copy()
 
@@ -90,6 +78,19 @@ def mpi_factory(env: 'Environment',
         candidates.append(functools.partial(
             MSMPIDependency, 'msmpi', env, kwargs, language=language))
 
+    # Only OpenMPI has pkg-config, and it doesn't work with the intel compilers
+    # for MPI, environment variables and commands like mpicc should have priority
+    if DependencyMethods.PKGCONFIG in methods and not compiler_is_intel:
+        pkg_name = None
+        if language == 'c':
+            pkg_name = 'ompi-c'
+        elif language == 'cpp':
+            pkg_name = 'ompi-cxx'
+        elif language == 'fortran':
+            pkg_name = 'ompi-fort'
+        candidates.append(functools.partial(
+            PkgConfigDependency, pkg_name, env, kwargs, language=language))
+
     return candidates
 
 packages['mpi'] = mpi_factory