[source] build: Suffix build directory with _$(LIBC) for external toolchains

[LEDE Commits]

jow pushed a commit to source.git, branch lede-17.01:
https://git.lede-project.org/3387158e45b1c0e329617d265392974b121230e7

commit 3387158e45b1c0e329617d265392974b121230e7
Author: Florian Fainelli <f.fainelli at gmail.com>
AuthorDate: Sat Jan 14 20:04:38 2017 -0800

    build: Suffix build directory with _$(LIBC) for external toolchains
    
    For external toolchain, we also know the type of C library used, and the
    toolchain triplet may not always be reflective of that, therefore make
    $(TARGET_DIR_NAME) suffixed with _$(LIBC).
    
    Signed-off-by: Florian Fainelli <f.fainelli at gmail.com>
    (cherry picked from commit 57657a72376000b5843367e627098f09fe9c3647)
---
 rules.mk | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/rules.mk b/rules.mk
index d58f8d9..1ed80f1 100644
--- a/rules.mk
+++ b/rules.mk
@@ -124,7 +124,8 @@ else
     GNU_TARGET_NAME=$(shell gcc -dumpmachine)
   endif
   REAL_GNU_TARGET_NAME=$(GNU_TARGET_NAME)
-  TARGET_DIR_NAME:=target-$(GNU_TARGET_NAME)$(if $(BUILD_SUFFIX),_$(BUILD_SUFFIX))
+  LIBC:=$(call qstrip,$(CONFIG_LIBC))
+  TARGET_DIR_NAME:=target-$(GNU_TARGET_NAME)_$(LIBC)$(if $(BUILD_SUFFIX),_$(BUILD_SUFFIX))
   TOOLCHAIN_DIR_NAME:=toolchain-$(GNU_TARGET_NAME)
 endif