Skip manpage and html doc installation if they weren't generated

Otherwise we get errors like this:

 CMake Error at doc/cmake_install.cmake:46 (file):
  file INSTALL cannot find
  "/<<PKGBUILDDIR>>/build/doc/html": No such file or
  directory.
 Call Stack (most recent call first):
  cmake_install.cmake:59 (include)

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Build
 - Fixed compiler warning in Clang 13 ([`#2613`](https://github.com/polybar/polybar/pull/2613))
 - Fixed compiler error in GCC 12 ([`#2616`](https://github.com/polybar/polybar/pull/2616), [`#2614`](https://github.com/polybar/polybar/issues/2614))
+- Fixed installation of docs when some are not generated (man, html...) ([`#2612`](https://github.com/polybar/polybar/pull/2612)
 
 ## [3.6.0] - 2022-03-01
 ### Breaking

doc/CMakeLists.txt

@@ -49,10 +49,12 @@ endforeach()
 # Dummy target that depends on all documentation targets
 add_custom_target(doc ALL DEPENDS ${doc_targets})
 
+if (BUILD_DOC_HTML)
   install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html/
   DESTINATION ${CMAKE_INSTALL_DOCDIR}
   COMPONENT doc
   PATTERN ".buildinfo" EXCLUDE)
+endif()
 
 install(FILES ${CMAKE_SOURCE_DIR}/CHANGELOG.md
   DESTINATION ${CMAKE_INSTALL_DOCDIR}
@@ -62,6 +64,7 @@ install(FILES ${CMAKE_CURRENT_LIST_DIR}/config.ini
   DESTINATION ${CMAKE_INSTALL_DOCDIR}/examples
   COMPONENT doc)
 
+if (BUILD_DOC_MAN)
   install(FILES ${CMAKE_CURRENT_BINARY_DIR}/man/polybar.1
   DESTINATION ${CMAKE_INSTALL_MANDIR}/man1
   COMPONENT doc)
@@ -73,3 +76,5 @@ install(FILES ${CMAKE_CURRENT_BINARY_DIR}/man/polybar-msg.1
   install(FILES ${CMAKE_CURRENT_BINARY_DIR}/man/polybar.5
   DESTINATION ${CMAKE_INSTALL_MANDIR}/man5
   COMPONENT doc)
+endif()
+