Add configuration option to enable SSE2.
Add -ffloat-store flag on x86 without SSE2.

Gentoo bug: https://bugs.gentoo.org/631852
Gentoo bug: https://bugs.gentoo.org/641716
Upstream bug: https://bugs.freedesktop.org/show_bug.cgi?id=101033

This patch is an aggregation of the following upstream commits:
5996bbd995aed5045cc22e4d1fab08c989377983
77bf71ea365a19ac55c59cf10399b566a02d82c1
939482ab2b5a6585bdd2e5251f3f2f113d64686f
cd617d181de03a7a13c2020e6c73cd14585e24b6
f136d434f0809e064ac195b5bc4e0b50484a474c

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e9a699e..d140a24 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,6 @@
 ######## Project settings
 cmake_minimum_required(VERSION 2.8.5)
+include(CheckCCompilerFlag)
 set (PACKAGE_NAME uchardet)
 project (${PACKAGE_NAME} CXX C)
 enable_testing()
@@ -33,13 +34,31 @@ include(GNUInstallDirs)

 ######## Configuration

-option(BUILD_BINARY "Build executable" ON)
-option(BUILD_SHARED_LIBS "Build shared library and link executable to it" ON)
+option(BUILD_BINARY "Build the CLI tool." ON)
+option(BUILD_SHARED_LIBS "Build shared library and link executable to it." ON)
+option(CHECK_SSE2 "Check and enable SSE2 extensions if supported. Disabling SSE on platforms which support it may decrease performances." ON)
+set(TARGET_ARCHITECTURE "" CACHE STRING "Target CPU architecture. It is autodetected if not specified.")

 if (BUILD_SHARED_LIBS)
 	option(BUILD_STATIC "Build static library" ON)
 endif (BUILD_SHARED_LIBS)

+if (TARGET_ARCHITECTURE STREQUAL "")
+    string(TOLOWER ${CMAKE_SYSTEM_PROCESSOR} TARGET_ARCHITECTURE)
+endif (TARGET_ARCHITECTURE STREQUAL "")
+
+if (TARGET_ARCHITECTURE MATCHES ".*(x86)|(amd).*")
+    CHECK_C_COMPILER_FLAG(-msse2 SUPPORTS_CFLAG_SSE2)
+    CHECK_C_COMPILER_FLAG(-mfpmath=sse SUPPORTS_CFLAG_SSE_MATH)
+    if (CHECK_SSE2 AND SUPPORTS_CFLAG_SSE2 AND SUPPORTS_CFLAG_SSE_MATH)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -msse2 -mfpmath=sse")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -msse2 -mfpmath=sse")
+    else (CHECK_SSE2 AND SUPPORTS_CFLAG_SSE2 AND SUPPORTS_CFLAG_SSE_MATH)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ffloat-store")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -ffloat-store")
+    endif (CHECK_SSE2 AND SUPPORTS_CFLAG_SSE2 AND SUPPORTS_CFLAG_SSE_MATH)
+endif (TARGET_ARCHITECTURE MATCHES ".*(x86)|(amd).*")
+
 configure_file(
 	uchardet.pc.in
 	uchardet.pc