Use CMake commands to set S2N_LIBCRYPTO environment variable

[boquan-fang]

Problem:

The current way of setting S2N_LIBCRYPTO environment variable is through passing it in CMAKE_PREFIX_PATH, and then use it in CMakeLists.txt. We want to set S2N_LIBCRYPTO in a more deliberate CMake design.

s2n-tls/codebuild/spec/buildspec_sanitizer.yml

Lines 104 to 109 in a29068a

	cmake . -Bbuild \
	-DCMAKE_C_COMPILER=/usr/bin/$COMPILER \
	-DCMAKE_PREFIX_PATH=/usr/local/$S2N_LIBCRYPTO \
	-DCMAKE_BUILD_TYPE=RelWithDebInfo \
	-DASAN=ON \
	-DUBSAN=ON

s2n-tls/CMakeLists.txt

Lines 235 to 238 in a29068a

	if (NOT $ENV{S2N_LIBCRYPTO} MATCHES "awslc")
	# add cast-qual back in for non AWS-LC
	target_compile_options(${PROJECT_NAME} PRIVATE -Wcast-qual)
	endif()

Solution:

• Pass libcrypto into CMake, and use set to set environment variable.

Requirements / Acceptance Criteria:

• Buildspec files shouldn't directly pass in libcrypto through CMAKE_PREFIX_PATH.
• CMakeLists.txt should directly set S2N_LIBCRYPTO via CMake commands.