Addressing OE linking issues when built with GCC

[mjp41]

This aims to address
openenclave/openenclave#2802

@jhand2, @anakrish, @achamayou could someone try this branch to see if it fixes the issues?

[achamayou]

@mjp41 Different error now with gcc-8/g++-8

[1653/2661] Linking CXX executable tests/tools/oecertdump/enc/oecertdump_enc
FAILED: tests/tools/oecertdump/enc/oecertdump_enc 
: && /usr/bin/g++-8  -g   tests/tools/oecertdump/enc/CMakeFiles/oecertdump_enc.dir/enc.cpp.o tests/tools/oecertdump/enc/CMakeFiles/oecertdump_enc.dir/oecertdump_t.c.o  -o tests/tools/oecertdump/enc/oecertdump_enc  output/lib/openenclave/enclave/liboeenclave.a output/lib/openenclave/enclave/liboeenclave.a output/lib/openenclave/enclave/liboelibc.a output/lib/openenclave/enclave/liboecryptombed.a 3rdparty/mbedtls/libmbedtls.a 3rdparty/mbedtls/libmbedx509.a 3rdparty/mbedtls/libmbedcrypto.a output/lib/openenclave/enclave/liboelibc.a syscall/liboesyscall.a output/lib/openenclave/enclave/liboecore.a -nostdlib -nodefaultlibs -nostartfiles -Wl,--no-undefined,-Bstatic,-Bsymbolic,--export-dynamic,-pie,--build-id -Wl,-z,noexecstack -Wl,-z,now -Wl,-gc-sections && :
output/lib/openenclave/enclave/liboecore.a(snmalloc_wrapper.cpp.o): In function `std::function<void* (void*)>::operator()(void*) const':
/usr/include/c++/8/bits/std_function.h:686: undefined reference to `std::__throw_bad_function_call()'
output/lib/openenclave/enclave/liboecore.a(snmalloc_wrapper.cpp.o):(.data.rel.ro._ZTIZN8snmalloc9AllocatorIXadL_ZNS_20needs_initialisationEPvEEXadL_ZNS_21init_thread_allocatorERSt8functionIFS1_S1_EEEENS_24MemoryProviderStateMixinINS_13PALPlainMixinINS_14PALOpenEnclaveEEEEENS_15DefaultChunkMapINS_21GlobalPagemapTemplateINS_7PagemapILm20EhLh0EEEEEEELb1EE23small_alloc_first_allocILNS_7ZeroMemE1ELNS_12AllowReserveE1EEES1_mEUlS1_E_[_ZTIZN8snmalloc9AllocatorIXadL_ZNS_20needs_initialisationEPvEEXadL_ZNS_21init_thread_allocatorERSt8functionIFS1_S1_EEEENS_24MemoryProviderStateMixinINS_13PALPlainMixinINS_14PALOpenEnclaveEEEEENS_15DefaultChunkMapINS_21GlobalPagemapTemplateINS_7PagemapILm20EhLh0EEEEEEELb1EE23small_alloc_first_allocILNS_7ZeroMemE1ELNS_12AllowReserveE1EEES1_mEUlS1_E_]+0x0): undefined reference to `vtable for __cxxabiv1::__class_type_info'

[achamayou]

Sounds like some undefined pure virtuals, possibly: https://gcc.gnu.org/faq.html#vtables

[mjp41]

So @plietar's patch in #166 removes the use of std::function. Can you cherry-pick his commit onto this PR, and see if it works?

[achamayou]

@mjp41, yes, that fixes it with gcc-8, and there's no regression with clang.

[achamayou]

gcc-9 is happy too.