Adding code-signing function

Meant only for MacOS/X, and mainly to avoid anoying pop-ups with MPI
programs.

scripts/AddCodeSigning.cmake

@@ -0,0 +1,38 @@
+# Adds the ability to do code signing for macos
+# It avoids getting a pop-up all the time when running MPI codes.
+# The process is as follows:
+#
+# Security nerds, avert your eyes.
+#
+# 1. create a *code-signing* certificate:
+#     - open key-chain access
+#     - select the login keychain (top-left panel)
+#     - menu KeyChain Access/Certificate Assistant/Create a certificate
+#     - choose a certificate name, self-signed, code-signing
+#     - curse Apple for not providing a command-line utility
+# 1. run cmake with:
+#
+#     ~~~bash
+#     cmake -DCERTIFICATE="certificate name" .
+#     ~~~
+#
+# 1. to every mpi target, add in the relevant CMakeLists.txt:
+#
+#     ~~~CMake
+#     add_codesigning(targetname)
+#     ~~~
+#
+# 1. the very first time the certificate is used (during build) click always allow
+
+set(CERTIFICATE "" CACHE STRING "Name of the certificate to use for code-signing")
+function(add_codesign target)
+  if(APPLE AND CERTIFICATE)
+    add_custom_command(
+      TARGET ${target}
+      POST_BUILD
+      COMMAND codesign -f -s "${CERTIFICATE}" $<TARGET_FILE:${target}>
+      COMMENT "Codsigning ${target}"
+      VERBATIM
+    )
+  endif()
+endfunction()