Allow linking without '-Wl,--as-needed', fix the build on clang+ld.bfd archs.
It's needed to do it like this since target_link_libraries takes preference
over LDFLAGS.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -72,6 +72,7 @@ set_verbose(FMT_INC_DIR ${CMAKE_INSTALL_INCLUDEDIR} CA
 option(FMT_PEDANTIC "Enable extra warnings and expensive tests." OFF)
 option(FMT_WERROR "Halt the compilation with an error on compiler warnings."
        OFF)
+option(AS_NEEDED "Use -Wl,--as-needed for linking." ON)
 
 # Options that control generation of various targets.
 option(FMT_DOC "Generate the doc target." ${FMT_MASTER_PROJECT})
@@ -286,7 +287,7 @@ endif ()
 
 if (BUILD_SHARED_LIBS)
   if (UNIX AND NOT APPLE AND NOT ${CMAKE_SYSTEM_NAME} MATCHES "SunOS" AND
-      NOT EMSCRIPTEN)
+      NOT EMSCRIPTEN AND AS_NEEDED)
     # Fix rpmlint warning:
     # unused-direct-shlib-dependency /usr/lib/libformat.so.1.1.0 /lib/libm.so.6.
     target_link_libraries(fmt -Wl,--as-needed)