rename and document script to automatically convert 'default(none)' to 'default(shared)' OpenMP pragmas

doc/src/Build_basics.txt

@@ -116,6 +116,18 @@ enables OpenMP. For GNU compilers it is -fopenmp.  For (recent) Intel
 compilers it is -qopenmp.  If you are using a different compiler,
 please refer to its documentation.
 
+[OpenMP Compiler compatibility info]: :link(default-none-issues)
+
+Some compilers do not fully support the 'default(none)' directive
+and others (e.g. GCC version 9 and beyond) may implement OpenMP 4.0
+semantics, which are incompatible with the OpenMP 3.1 directives used
+in LAMMPS (for maximal compatibility with compiler versions in use).
+In those case, all 'default(none)' directives (which aid in detecting
+incorrect and unwanted sharing) can be replaced with 'default(shared)'
+while dropping all 'shared()' directives. The script
+'src/USER-OMP/hack_openmp_for_pgi_gcc9.sh' can be used to automate
+this conversion.
+
 :line
 
 Choice of compiler and compile/link options :h4,link(compile)

doc/src/Build_extras.txt

@@ -932,7 +932,9 @@ LINKFLAGS: -fopenmp             # for GNU Compilers
 LINKFLAGS: -qopenmp             # for Intel compilers on Linux :pre
 
 For other platforms and compilers, please consult the documentation
-about OpenMP support for your compiler.
+about OpenMP support for your compiler. Please see the note about
+how to address compatibility "issues with the 'default(none)'
+directive"_Build_basics.html#default-none-issues of some compilers.
 
 :line
 

src/USER-OMP/hack_openmp_for_pgi.sh

@@ -1,8 +0,0 @@
-#!/bin/sh
-
-for f in *.h *.cpp
-do \
-   sed -e '/#pragma omp/s/^\(.*default\)(none)\(.*\)$/\1(shared)\2/' \
-       -e '/#pragma omp/s/shared([a-z0-9,_]\+)//' \
-       -i.bak $f
-done

src/USER-OMP/hack_openmp_for_pgi_gcc9.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+# convert default(none) directives for OpenMP pragmas to default(shared) and remove shared() directive
+# this allows compiling OpenMP pragmas in LAMMPS with compilers that don't support default(none) properly
+# or require backward incompatible OpenMP 4 and OpenMP 5 semantics
+
+for f in *.h *.cpp
+do \
+   sed -e '/#pragma omp/s/^\(.*default\)(none)\(.*\)$/\1(shared)\2/' \
+       -e '/#pragma omp/s/shared([a-z0-9,_]\+)//' \
+       -i.bak $f
+done