Fix the g++ build.

It turns out that the g++ test build, initiated by "make presubmit", was
actually using clang++ for the build.  This fixes the makefile, plus the
code that actually produces errors under the g++ build.

BUG= none
TBR=jpp

Review URL: https://codereview.chromium.org/1572863003 .
diff --git a/src/IceELFObjectWriter.cpp b/src/IceELFObjectWriter.cpp
index cb36e3d..611121a 100644
--- a/src/IceELFObjectWriter.cpp
+++ b/src/IceELFObjectWriter.cpp
@@ -330,7 +330,7 @@
   case ROData: {
     const IceString SectionName =
         MangleSectionName(IsPIC ? ".data.rel.ro" : ".rodata", SectionSuffix);
-    const Elf64_Xword ShFlags = SHF_ALLOC | (IsPIC ? SHF_WRITE : 0);
+    const Elf64_Xword ShFlags = IsPIC ? (SHF_ALLOC | SHF_WRITE) : SHF_ALLOC;
     Section = createSection<ELFDataSection>(SectionName, SHT_PROGBITS, ShFlags,
                                             ShAddralign, ShEntsize);
     Section->setFileOffset(alignFileOffset(ShAddralign));