cmake: consolidate libm library handling

Move the handling to the top-level, adding it _once_ to EXTRA_LIBS.
Thus avoiding some duplication across the board.

This change technically adds an extra link for the unit tests, which
seemingly was omitted by mistake. Alternatively, the extra over-linking
won't be an issue in practise.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/gui/curses/CMakeLists.txt

@@ -37,8 +37,6 @@ if(NOT ${CMAKE_SYSTEM_NAME} STREQUAL "Haiku")
   list(APPEND EXTRA_LIBS "pthread")
 endif()
 
-list(APPEND EXTRA_LIBS "m")
-
 if(ENABLE_NCURSES)
   subdirs(normal)
 endif()