gnucash master: Link with libm.so on those platforms that require it.

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash/commit/4ee573e2 (commit)
	from  https://github.com/Gnucash/gnucash/commit/43749a94 (commit)



commit 4ee573e23a15691247b509c981258a12bdd49194
Author: John Ralls <jralls at ceridwen.us>
Date:   Mon Jul 6 12:24:49 2020 -0700

    Link with libm.so on those platforms that require it.



Summary of changes:
 common/cmake_modules/CMakeLists.txt    |  2 +-
 common/cmake_modules/GncFindLibm.cmake | 46 ++++++++++++++++++++++++++++++++++
 libgnucash/app-utils/CMakeLists.txt    | 15 +++++++++--
 3 files changed, 60 insertions(+), 3 deletions(-)
 create mode 100644 common/cmake_modules/GncFindLibm.cmake