Makefiles: Avoid nested make calls for Fortran module dependencies

Makefiles generated by cmake use a series of nested calls to build
`*.provides.build` targets that are used when the 'requires' step is
needed.  That leads to significant degradation of the build time for
incremental builds.  Re-arrange dependencies to eliminate the nested
calls.

Explicit `.mod.stamp` targets introduced by this commit could lead to
situation when a stamp file always older than its dependency.  This
happens during the incremental build when building of an updated Fortran
source produces a module file that has no differences from the stored
stamp file.  In such case `cmake_copy_f90_mod` will be triggered on each
new build to compare a module file with the corresponding stamp file.
This behavior is expected and can not be changed without nested calls
that slow down the build.  The copy-if-different check is much cheaper
than an entire nested make call.

1 files changed, 2 insertions, 0 deletions

diff --git a/Source/cmLocalUnixMakefileGenerator3.cxx b/Source/cmLocalUnixMakefileGenerator3.cxx
index e26182a..a389ad0 100644
--- a/Source/cmLocalUnixMakefileGenerator3.cxx
+++ b/Source/cmLocalUnixMakefileGenerator3.cxx
@@ -1444,6 +1444,8 @@ bool cmLocalUnixMakefileGenerator3::ScanDependencies(
     }
 #ifdef CMAKE_BUILD_WITH_CMAKE
     else if (lang == "Fortran") {
+      ruleFileStream << "# Note that incremental build could trigger "
+                     << "a call to cmake_copy_f90_mod on each re-build\n";
       scanner = new cmDependsFortran(this);
     } else if (lang == "Java") {
       scanner = new cmDependsJava();