Add -DOPENSSL_SMALL to CMake.

Adding preprocessor flags requires a lot of typing in the CMake
command-line (-DCMAKE_C_FLAGS=-DOPENSSL_SMALL
-DCMAKE_CXX_FLAGS=-DOPENSSL_SMALL).

Change-Id: Ieafc4155d656306c1f22746f780faa5c1d3e27be
Reviewed-on: https://boringssl-review.googlesource.com/26784
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

BUILDING.md

@@ -150,7 +150,14 @@ corresponding ARM feature.
 Note that if a feature is enabled in this way, but not actually supported at
 run-time, BoringSSL will likely crash.
 
-# Running tests
+## Binary Size
+
+The implementations of some algorithms require a trade-off between binary size
+and performance. For instance, BoringSSL's fastest P-256 implementation uses a
+148 KiB pre-computed table. To optimize instead for binary size, pass
+`-DOPENSSL_SMALL=1` to CMake or define the `OPENSSL_SMALL` preprocessor symbol.
+
+# Running Tests
 
 There are two sets of tests: the C/C++ tests and the blackbox tests. For former
 are built by Ninja and can be run from the top-level directory with `go run

CMakeLists.txt

@@ -279,6 +279,10 @@ if(FIPS)
   endif()
 endif()
 
+if(OPENSSL_SMALL)
+  add_definitions(-DOPENSSL_SMALL)
+endif()
+
 # CMake's iOS support uses Apple's multiple-architecture toolchain. It takes an
 # architecture list from CMAKE_OSX_ARCHITECTURES, leaves CMAKE_SYSTEM_PROCESSOR
 # alone, and expects all architecture-specific logic to be conditioned within