newlib.mk: sanitize output before getting include path

Only keep lines that are indeed include path.
It also keeps newlines as they do not matter.

It fixes Mingw32 support where `grep '^\s'` is not working the same way.
It also handles some mac `sed` that do not support `\s`.

Ouput tested with:

    make -C examples/hello-world BOARD=samr21-xpro info-debug-variable-COMPILER_INCLUDE_PATHS
    # by also putting newlines for readability

Now:

    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include-fixed
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/../../../../arm-none-eabi/include

Before:

    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/cc1 -E -quiet -v -iprefix /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/ -isysroot /usr/bin/../arm-none-eabi -D__USES_INITFINI__ /dev/null
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include-fixed
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/../../../../arm-none-eabi/include

makefiles/libc/newlib.mk

@@ -23,7 +23,11 @@ export LINKFLAGS += -lc
 
 # Try to search for newlib in the standard search path of the compiler for includes
 ifeq (,$(NEWLIB_INCLUDE_DIR))
-  COMPILER_INCLUDE_PATHS := $(shell $(PREFIX)gcc -v -x c -E /dev/null 2>&1 | grep '^\s' | tr -d '\n')
+  COMPILER_INCLUDE_PATHS := $(shell $(PREFIX)gcc -v -x c -E /dev/null 2>&1 | \
+                              sed \
+                              -e '1,/\#include <...> search starts here:/d' \
+                              -e '/End of search list./,$$d' \
+                              -e 's/^ *//')
   NEWLIB_INCLUDE_DIR := $(firstword $(dir $(wildcard $(addsuffix /newlib.h, $(COMPILER_INCLUDE_PATHS)))))
 endif