Bug 1290616 - Pass down LD to the NSS build system. r=gps

as opposed to passing down LINK when cross compiling.
Until bug 1290334, LINK was the variable used by the NSS build system
for the linker on Windows, but it now uses LD, like on other platforms.

But its default value on Windows is "link", which can pick /usr/bin/link
on msys2, instead of MSVC's so, pass down LD from the Gecko build
system. And do so an all platforms in all cases, because it shouldn't
hurt.

config/external/nss/Makefile.in

@@ -118,6 +118,7 @@ export MOZ_DEBUG_SYMBOLS
 DEFAULT_GMAKE_FLAGS =
 DEFAULT_GMAKE_FLAGS += CC='$(CC)'
 DEFAULT_GMAKE_FLAGS += MT='$(MT)'
+DEFAULT_GMAKE_FLAGS += LD='$(LD)'
 DEFAULT_GMAKE_FLAGS += SOURCE_MD_DIR=$(ABS_DIST)
 DEFAULT_GMAKE_FLAGS += SOURCE_MDHEADERS_DIR=$(NSPR_INCLUDE_DIR)
 DEFAULT_GMAKE_FLAGS += DIST=$(ABS_DIST)
@@ -201,7 +202,6 @@ DEFAULT_GMAKE_FLAGS += \
 	NATIVE_CC='$(HOST_CC)' \
 	CC='$(CC)' \
 	CCC='$(CXX)' \
-	LINK='$(LD)' \
 	AS='$(AS)' \
 	AR='$(AR) $(AR_FLAGS:$@=$$@)' \
 	RANLIB='$(RANLIB)' \