Add separate cmake file for setting OS variables

In the Options.cmake file, MAP_TEXT_HUGE_PAGES and ENABLE_FOLLY_SYMBOLIZER
are set depending on the value of the LINUX variable, which is only set in
a file included later - HPHPCompiler.cmake. This causes these 2 variables not
to be visible in third-party modules the first time configure is run
(subsequent times the variables are taken from the cmake cache).

This change fixes this behavior by adding a new cmake file that sets the OS
variables and including it before Options.cmake.

Author: Florin Papa

CMake/HPHPCompiler.cmake

@@ -1,20 +1,3 @@
-set(FREEBSD FALSE)
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "FreeBSD")
-  set(FREEBSD TRUE)
-endif()
-set(LINUX FALSE)
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
-  set(LINUX TRUE)
-endif()
-set(DARWIN FALSE)
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
-  set(DARWIN TRUE)
-endif()
-set(WINDOWS FALSE)
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
-  set(WINDOWS TRUE)
-endif()
-
 # Do this until cmake has a define for ARMv8
 INCLUDE(CheckCXXSourceCompiles)
 CHECK_CXX_SOURCE_COMPILES("

CMake/SetOSVars.cmake

@@ -0,0 +1,16 @@
+set(FREEBSD FALSE)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "FreeBSD")
+  set(FREEBSD TRUE)
+endif()
+set(LINUX FALSE)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
+  set(LINUX TRUE)
+endif()
+set(DARWIN FALSE)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
+  set(DARWIN TRUE)
+endif()
+set(WINDOWS FALSE)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
+  set(WINDOWS TRUE)
+endif()

CMakeLists.txt

@@ -108,6 +108,7 @@ SET(TP_DIR "${CMAKE_CURRENT_SOURCE_DIR}/third-party")
 SET(TP_BUILD_DIR "${CMAKE_CURRENT_BINARY_DIR}/third-party")
 
 include(MSVCDefaults)
+include(SetOSVars)
 include(Options)
 include(HPHPCompiler)
 include(HPHPFindLibs)