Generate CMake export set regardless of FMT_INSTALL (#4850)

export() was scoped inside if (FMT_INSTALL), so projects that pull in
fmt via add_subdirectory()/FetchContent without installing it had no
way to get the exported fmt::* targets. If such a project tries to
export its own targets that depend on fmt, CMake fails with "target
... requires target fmt that is not in any export set".

Hoist the target list/export name and the export() call itself out of
the FMT_INSTALL guard so the build-tree export file is always
generated; the install()-only pieces (config/version files, pkgconfig,
install(EXPORT ...)) stay behind the guard and installed behavior is
unchanged.

Fixes #4806

Co-authored-by: flink <w741069229@gmail.com>

test/add-subdirectory-test/CMakeLists.txt

@@ -15,3 +15,12 @@ if (TARGET fmt::fmt-header-only)
   target_compile_options(header-only-test PRIVATE ${PEDANTIC_COMPILE_FLAGS})
   target_link_libraries(header-only-test fmt::fmt-header-only)
 endif ()
+
+# Test that a target publicly depending on fmt can itself be exported even
+# though fmt is added via add_subdirectory and FMT_INSTALL is off (#4806).
+add_library(export-test STATIC export-lib.cc)
+target_link_libraries(export-test PUBLIC fmt::fmt)
+export(
+  TARGETS export-test
+  NAMESPACE test::
+  FILE ${CMAKE_CURRENT_BINARY_DIR}/export-test-targets.cmake)

test/add-subdirectory-test/export-lib.cc

@@ -0,0 +1,3 @@
+#include "fmt/base.h"
+
+void greet(const char* name) { fmt::print("Hello, {}!\n", name); }