scripts: allow skipping elfutils rebuild in build-fuzzers.sh

This simplifies local reproduction of fuzzer reported errors.
E.g. the following sequence of commands would execute much faster on a
second run:

  $ SKIP_LIBELF_REBUILD=1 scripts/build-fuzzers.sh
  $ out/bpf-object-fuzzer <path-to-test-case>

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

scripts/build-fuzzers.sh

@@ -35,11 +35,14 @@ if [[ "$SANITIZER" == undefined ]]; then
     CXXFLAGS+=" $UBSAN_FLAGS"
 fi
 
+export SKIP_LIBELF_REBUILD=${SKIP_LIBELF_REBUILD:=''}
+
 # Ideally libbelf should be built using release tarballs available
 # at https://sourceware.org/elfutils/ftp/. Unfortunately sometimes they
 # fail to compile (for example, elfutils-0.185 fails to compile with LDFLAGS enabled
 # due to https://bugs.gentoo.org/794601) so let's just point the script to
 # commits referring to versions of libelf that actually can be built
+if [[ ! -e elfutils || "$SKIP_LIBELF_REBUILD" == "" ]]; then
     rm -rf elfutils
     git clone https://sourceware.org/git/elfutils.git
     (
@@ -72,6 +75,7 @@ make -C config -j$(nproc) V=1
         make -C lib -j$(nproc) V=1
         make -C libelf -j$(nproc) V=1
     )
+fi
 
 make -C src BUILD_STATIC_ONLY=y V=1 clean
 make -C src -j$(nproc) CFLAGS="-I$(pwd)/elfutils/libelf $CFLAGS" BUILD_STATIC_ONLY=y V=1