bug#27866: Handle clang's internal libraries when finding compiler's internal libraries

Hi,

This is Manoj working on ChromeOS. I am facing a problem trying to build it
with clang with its own internal library (compiler-rt) since some packages
like mesa fail to build. The root cause is clang uses an absolute path to
link its internal libraries which libtool does not recognize.

e.g. clang++ -rtlib=compiler-rt main.cpp -v shows use of
/usr/lib64/clang/5.0.0/lib/linux/libclang_rt.builtins-x86_64.a

Libtool currently relies on "-lname" pattern to find the internal
libraries. And this does not work if some code is compiled using +
compiler-rt.
The issue was discovered in building mesa graphics library which uses
-nostdlib flag  and relies on libtool to pass the additionally required
compiler internal libraries.

I have a sample fix below for fixing this for clang.

+--- a/m4/libtool.m4
++++ b/m4/libtool.m4
+@@ -7531,7 +7544,7 @@
+   for p in `eval "$output_verbose_link_cmd"`; do
+     case $prev$p in
+
+-    -L* | -R* | -l*)
++    -L* | -R* | -l* | *clang_rt*.a)
+        # Some compilers place space between "-{L,R}" and the path.
+        # Remove the space.
+        if test x-L = "$p" ||
+

Please let me know if this is an appropriate fix.

Thanks,
Manoj

Sample linker command line when called by clang with compiler-rt:

 "/usr/bin/x86_64-pc-linux-gnu-ld" --build-id --eh-frame-hdr -m elf_x86_64
-dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/../../../../lib64/crt1.o
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/../../../../lib64/crti.o
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/crtbegin.o
-L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x
-L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/../../../../lib64
-L/usr/bin/../lib64 -L/lib/../lib64 -L/usr/lib/../lib64
-L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/../../../../x86_64-pc-linux-gnu/lib
-L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/../../..
-L/usr/bin/../lib -L/lib -L/usr/lib /tmp/main-6b0bb5.o -lc++ -lm
/usr/lib64/clang/5.0.0/lib/linux/libclang_rt.builtins-x86_64.a -lgcc_eh -lc
/usr/lib64/clang/5.0.0/lib/linux/libclang_rt.builtins-x86_64.a -lgcc_eh
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/crtend.o
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/../../../../lib64/crtn.o


Thanks,
Manoj

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