Process compiler.libraries.ldflags (#3783)

Add support for the v1.8.6 compiler.libraries.ldflags:

https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification

This fixes the issue reported here:

https://community.bosch-sensortec.com/t5/MEMS-sensors-forum/BSEC-1-4-7-4-compilation-error-on-ESP32-1-0-3-rc1/td-p/9120

Author: mcdeoliveira

Diff for: platform.txt

@@ -60,6 +60,7 @@ compiler.cpp.extra_flags=
 compiler.ar.extra_flags=
 compiler.objcopy.eep.extra_flags=
 compiler.elf2hex.extra_flags=
+compiler.libraries.ldflags=
 
 # Build Dir: {build.path}
 # Sketch Dir: {build.source.path}
@@ -81,7 +82,7 @@ recipe.S.o.pattern="{compiler.path}{compiler.c.cmd}" {compiler.cpreprocessor.fla
 recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}"
 
 ## Combine gc-sections, archives, and objects
-recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.flags} {compiler.c.elf.extra_flags} -Wl,--start-group {object_files} "{archive_file_path}" {compiler.c.elf.libs} -Wl,--end-group -Wl,-EL -o "{build.path}/{build.project_name}.elf"
+recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.flags} {compiler.c.elf.extra_flags} -Wl,--start-group {object_files} "{archive_file_path}" {compiler.c.elf.libs} {compiler.libraries.ldflags} -Wl,--end-group -Wl,-EL -o "{build.path}/{build.project_name}.elf"
 
 ## Create eeprom
 recipe.objcopy.eep.pattern={tools.gen_esp32part.cmd} -q "{build.path}/partitions.csv" "{build.path}/{build.project_name}.partitions.bin"