Ensure changes to CMAKE_INSTALL_INCLUDEDIR are respected for [1]

Note headers are installed twice (overwritten), CMakeLists.txt's
does not respect INCLUDEDIR while cmake/SlangTarget.cmake does.

[1] https://bugs.gentoo.org/980471
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -660,5 +659,0 @@
-install(
-    DIRECTORY "${slang_SOURCE_DIR}/include"
-    DESTINATION .
-    PATTERN ".*" EXCLUDE
-)
--- a/extras/pkgconfig/slang-compiler.pc.in
+++ b/extras/pkgconfig/slang-compiler.pc.in
@@ -4 +4 @@
-includedir=${prefix}/include
+includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@