Use ${CMAKE_C_FLAGS_${BUILD_TYPE_UPPER}} to gain some performance

[moi15moi]

When compiling with android studio, android studio call set CMAKE_TOOLCHAIN_FILE to ${ANDROID_NDK_ROOT}/build/cmake/android.toolchain.cmake

This file import android-legacy.toolchain.cmake and I realised that depending on the build type, it is setting different variant of CFLAGS, CXXFLAGS and LDFLAGS.

This is their values (CMAKE_CXX_FLAGS_XXX is the same as CMAKE_C_FLAGS_XXX):

CMAKE_C_FLAGS_RELEASE: -O3 -DNDEBUG
CMAKE_C_FLAGS_DEBUG: -fno-limit-debug-info
CMAKE_C_FLAGS_RELWITHDEBINFO: -O2 -g -DNDEBUG

CMAKE_SHARED_LINKER_FLAGS_RELEASE: -Wl,--gc-sections
CMAKE_SHARED_LINKER_FLAGS_DEBUG is empty
CMAKE_SHARED_LINKER_FLAGS_RELWITHDEBINFO: -Wl,--gc-sections

Unfortunatly, CMAKE_C_FLAGS always contains the "-g" flags, so even release build have symbol:

-g -DANDROID -fdata-sections -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security

PS: CMAKE_CXX_FLAGS is equal to CMAKE_C_FLAGS.

[moi15moi]

By default, it seems that if CMAKE_BUILD_TYPE isn't specified, it is empty.
I should verify if android studio specify a CMAKE_BUILD_TYPE or not.
Currently, with the CI, we don't specify CMAKE_BUILD_TYPE, so it is empty so ${CMAKE_C_FLAGS_${BUILD_TYPE_UPPER}} doesn't inject anything.