Add unistd.h checks back to the CMake build.

getpagesize(), as well as its POSIX.2001 replacement
sysconf(_SC_PAGESIZE), is defined in <unistd.h>. On Linux and OS X,
including <sys/mman.h> is sufficient to get a definition for
getpagesize(). However, this is not true for the Android NDK. This CL
brings back the HAVE_UNISTD_H definition and its associated header
check.

This also adds a HAVE_FUNC_SYSCONF definition, which checks for the
presence of sysconf(). The definition can be used later to replace
getpagesize() with sysconf().

CMakeLists.txt

@@ -22,6 +22,7 @@ check_include_file("sys/mman.h" HAVE_SYS_MMAN_H)
 check_include_file("sys/resource.h" HAVE_SYS_RESOURCE_H)
 check_include_file("sys/time.h" HAVE_SYS_TIME_H)
 check_include_file("sys/uio.h" HAVE_SYS_UIO_H)
+check_include_file("unistd.h" HAVE_UNISTD_H)
 check_include_file("windows.h" HAVE_WINDOWS_H)
 
 include(CheckLibraryExists)
@@ -37,6 +38,7 @@ check_cxx_source_compiles(
 
 include(CheckSymbolExists)
 check_symbol_exists("mmap" "sys/mman.h" HAVE_FUNC_MMAP)
+check_symbol_exists("sysconf" "unistd.h" HAVE_FUNC_SYSCONF)
 
 find_package(GTest QUIET)
 if(GTEST_FOUND)