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Install modulemap along with other header files

See merge request !6

Fix tiny CMake syntax error in ClhepToolchain.cmake

See merge request !7

This error is visible in case the CHLEP is compiled with cmake and the resulting pkgconfig is used elsewhere.

Vassil Vassilev authored May 23, 2020 and Lynn Garren committed Nov 09, 2020

The C++ modules feature as described in https://clang.llvm.org/docs/Modules.html
allow producing a binary header representation to avoid redundant header
reparsing.

This feature is used in ROOT's dictionary system since ROOT v6.20:
https://github.com/root-project/root/blob/master/README/README.CXXMODULES.md

CMSSW and other experiment migrate their dictionaries to use the provided by
ROOT C++ modules support: https://github.com/cms-sw/cmssw/issues/15248

Dictionaries which transiently include clhep can be further optimized by
building a separate module for CLHEP which this MR aims for.

The current patch introduces a module.modulemap file containing a mapping between
a binary artifact (a module or a pcm file) and a set of header files. The C++
modules are more picky on translation unit encapsulation and thus require all
headers which a translation unit uses to be included. In addition to the
missing include we outline a few virtual destructors to avoid pollution of .o
files with weak virtual tables.

This patch enables builds with modules via rootcling/genreflex/rootcint and
enables compile-time module builds if configured like:
cmake -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CXX_FLAGS="-fmodules -Xclang -fmodules-local-submodule-visibility" ../clhep/

The module.modulemap file is easy to maintain as it globs for the well-behaved
header files and enumerates already the ones needed special treatment.

Once merged we will backport this to cmssw.

Conflicts:
	Matrix/src/DiagMatrix.cc
	Matrix/src/GenMatrix.cc
	Matrix/src/Matrix.cc
	Matrix/src/SymMatrix.cc
	Matrix/src/Vector.cc

Note: The substitution of x()/y()/z() in operator() is necessary to
avoid infinite recursion, as x()/y()/z() are now defined in terms of
operator[] instead.

This is in preparation to change the data layout of Hep3Vector
by replacing the private dx/dy/dz members with a vector, data[3],
to allow us to remove the inefficient switch from operator[]().

These pragmas are obsolete since GCC 2.7.2.
https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Interface.html.

- Run doxygen -u doxygen.conf.in
- Map .icc file extension to C++
- Add custom FILE_PATTERNS to process *.icc inline headers

CLHEP 2.4.1.3 with move constructors CLHEP_2_4_1_3

CLHEP 2.4.1.0 CLHEP_2_4_1_0