GNUInstallDirs: Fix double separator in /opt special-case absolute paths

Since commit 7566a477f7 (GNUInstallDirs: Factor out helper for special
absolute path logic, 2025-03-27, v4.1.0), when CMAKE_INSTALL_PREFIX is
under /opt/ (excluding /opt/homebrew), the special case FHS directories
include an extra path separator, resulting in values like /etc//opt/foo.
This is observed in CMAKE_INSTALL_FULL_SYSCONFDIR,
CMAKE_INSTALL_FULL_LOCALSTATEDIR, and CMAKE_INSTALL_FULL_RUNSTATEDIR
whenever their corresponding CMAKE_INSTALL_<dir> may be relative, as is
the case under policy CMP0192 OLD or if set explicitly while under
CMP0192 NEW (e.g. -DCMAKE_INSTALL_SYSCONFDIR=etc). Restore the previous
path calculation, which relies on the prefix starting with `/`.

Fixes: #28186

Modules/GNUInstallDirs.cmake

@@ -360,7 +360,7 @@ function(_GNUInstallDirs_special_absolute out_var original_path install_prefix)
   if(install_prefix MATCHES "^/usr/?$")
     set(${out_var} "/${original_path}")
   elseif(install_prefix MATCHES "^/opt/" AND NOT install_prefix MATCHES "^/opt/homebrew/")
-    set(${out_var} "/${original_path}/${install_prefix}")
+    set(${out_var} "/${original_path}${install_prefix}")
   endif()
 
   return(PROPAGATE ${out_var})

Tests/RunCMake/GNUInstallDirs/Opt-CMP0192-OLD-stderr.txt

@@ -0,0 +1,3 @@
+^CMAKE_INSTALL_FULL_LOCALSTATEDIR='/var/opt/Opt-CMP0192-OLD'
+CMAKE_INSTALL_FULL_RUNSTATEDIR='/var/run/opt/Opt-CMP0192-OLD'
+CMAKE_INSTALL_FULL_SYSCONFDIR='/etc/opt/Opt-CMP0192-OLD'$

Tests/RunCMake/GNUInstallDirs/Opt-CMP0192-OLD.cmake

@@ -0,0 +1,7 @@
+enable_language(C)
+cmake_policy(SET CMP0192 OLD)
+set(CMAKE_INSTALL_PREFIX "/opt/${PROJECT_NAME}")
+include(GNUInstallDirs)
+foreach(dir IN ITEMS LOCALSTATEDIR RUNSTATEDIR SYSCONFDIR)
+  message("CMAKE_INSTALL_FULL_${dir}='${CMAKE_INSTALL_FULL_${dir}}'")
+endforeach()

Tests/RunCMake/GNUInstallDirs/Opt-RelativeSpecialDirs-stderr.txt

@@ -0,0 +1,3 @@
+^CMAKE_INSTALL_FULL_LOCALSTATEDIR='/var/opt/Opt-RelativeSpecialDirs'
+CMAKE_INSTALL_FULL_RUNSTATEDIR='/var/run/opt/Opt-RelativeSpecialDirs'
+CMAKE_INSTALL_FULL_SYSCONFDIR='/etc/opt/Opt-RelativeSpecialDirs'$

Tests/RunCMake/GNUInstallDirs/Opt-RelativeSpecialDirs.cmake

@@ -0,0 +1,7 @@
+enable_language(C)
+cmake_policy(SET CMP0192 NEW)
+set(CMAKE_INSTALL_PREFIX "/opt/${PROJECT_NAME}")
+include(GNUInstallDirs)
+foreach(dir IN ITEMS LOCALSTATEDIR RUNSTATEDIR SYSCONFDIR)
+  message("CMAKE_INSTALL_FULL_${dir}='${CMAKE_INSTALL_FULL_${dir}}'")
+endforeach()

Tests/RunCMake/GNUInstallDirs/RunCMakeTest.cmake

@@ -39,6 +39,12 @@ endblock()
 
 run_cmake(GetAbs)
 run_cmake(NoSystem)
+run_cmake(Opt-CMP0192-OLD)
+run_cmake_with_options(Opt-RelativeSpecialDirs
+  -DCMAKE_INSTALL_LOCALSTATEDIR=var
+  -DCMAKE_INSTALL_RUNSTATEDIR=var/run
+  -DCMAKE_INSTALL_SYSCONFDIR=etc
+)
 
 foreach(case
     Opt