Add BUILD_SERVER similar to BUILD_VIEWER

Add a BUILD_SERVER option similar to BUILD_VIEWER.  When BUILD_SERVER is
not active, we do not require pam, so also disable everything that needs
pam.  This allows building the viewer on a machine where pam is not
installed.

See https://bugs.gentoo.org/852830

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -278,6 +278,7 @@ if(BUILD_JAVA)
   add_subdirectory(java)
 endif()
 
+trioption(BUILD_SERVER "Build TigerVNC server")
 trioption(BUILD_VIEWER "Build TigerVNC viewer")
 if(BUILD_VIEWER)
   # Check for FLTK
@@ -359,7 +360,7 @@ if (ENABLE_NETTLE)
 endif()
 
 # Check for PAM library
-if(UNIX AND NOT APPLE)
+if(BUILD_SERVER AND UNIX AND NOT APPLE)
   check_include_files(security/pam_appl.h HAVE_PAM_H)
   set(CMAKE_REQUIRED_LIBRARIES -lpam)
   check_function_exists(pam_start HAVE_PAM_START)
--- a/common/rfb/CMakeLists.txt
+++ b/common/rfb/CMakeLists.txt
@@ -75,7 +75,7 @@ if(WIN32)
   target_sources(rfb PRIVATE WinPasswdValidator.cxx)
 endif(WIN32)
 
-if(UNIX AND NOT APPLE)
+if(BUILD_SERVER AND UNIX AND NOT APPLE)
   target_sources(rfb PRIVATE UnixPasswordValidator.cxx)
   target_link_libraries(rfb ${PAM_LIBS})
 endif()
--- a/tests/perf/CMakeLists.txt
+++ b/tests/perf/CMakeLists.txt
@@ -8,8 +8,10 @@ target_link_libraries(convperf test_util rfb)
 add_executable(decperf decperf.cxx)
 target_link_libraries(decperf test_util rdr rfb)
 
-add_executable(encperf encperf.cxx)
-target_link_libraries(encperf test_util core rdr rfb)
+if(BUILD_SERVER)
+  add_executable(encperf encperf.cxx)
+  target_link_libraries(encperf test_util core rdr rfb)
+endif()
 
 if (BUILD_VIEWER)
   add_executable(fbperf
--- a/unix/CMakeLists.txt
+++ b/unix/CMakeLists.txt
@@ -4,7 +4,9 @@ endif()
 
 add_subdirectory(tx)
 add_subdirectory(common)
-add_subdirectory(vncconfig)
-add_subdirectory(vncpasswd)
-add_subdirectory(vncserver)
-add_subdirectory(x0vncserver)
+if(BUILD_SERVER)
+  add_subdirectory(vncconfig)
+  add_subdirectory(vncpasswd)
+  add_subdirectory(vncserver)
+  add_subdirectory(x0vncserver)
+endif()