Export: Prefix relative items with genexes in INSTALL_INTERFACE.

Code such as

 target_include_directories(foo INTERFACE
   $<INSTALL_INTERFACE:include$<FOO>>
 )

should be treated as a relative directory, despite the genex, after
the INSTALL_INTERFACE is stripped away.

Previously, this would generate a relative directory on export, which
would be an error on import, so no policy is needed.

Source/cmGeneratorExpression.cxx

@@ -245,7 +245,7 @@ static void prefixItems(const std::string &content, std::string &result,
     result += sep;
     sep = ";";
     if (!cmSystemTools::FileIsFullPath(ei->c_str())
-        && cmGeneratorExpression::Find(*ei) == std::string::npos)
+        && cmGeneratorExpression::Find(*ei) != 0)
       {
       result += prefix;
       }

Tests/ExportImport/Export/CMakeLists.txt

@@ -327,6 +327,8 @@ target_include_directories(testLibRequired INTERFACE
   $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest5$<0:>>
   $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/$<0:>installIncludesTest6>
   $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest7>
+  $<INSTALL_INTERFACE:installIncludesTest8/$<0:>>
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest8$<0:>>
 )
 install(TARGETS
           testLibIncludeRequired1
@@ -360,6 +362,8 @@ file(MAKE_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest6")
 file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest6/installIncludesTest6.h" "// No content\n")
 file(MAKE_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest7")
 file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest7/installIncludesTest7.h" "// No content\n")
+file(MAKE_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest8")
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest8/installIncludesTest8.h" "// No content\n")
 install(FILES
   "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest/installIncludesTest.h"
   DESTINATION installIncludesTest
@@ -388,6 +392,10 @@ install(FILES
   "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest7/installIncludesTest7.h"
   DESTINATION installIncludesTest7
 )
+install(FILES
+  "${CMAKE_CURRENT_BINARY_DIR}/installIncludesTest8/installIncludesTest8.h"
+  DESTINATION installIncludesTest8
+)
 
 install(TARGETS testLibDepends testSharedLibDepends EXPORT DependsExp DESTINATION lib )
 install(EXPORT DependsExp FILE testLibDependsTargets.cmake DESTINATION lib/cmake/testLibDepends)

Tests/ExportImport/Import/A/deps_iface.c

@@ -11,6 +11,7 @@
 #include "installIncludesTest5.h"
 #include "installIncludesTest6.h"
 #include "installIncludesTest7.h"
+#include "installIncludesTest8.h"
 
 #ifndef testLibRequired_IFACE_DEFINE
 #error Expected testLibRequired_IFACE_DEFINE