fix: refer to the right CMake module for Exiv2

The exiv2 library provides its own CMake config module, which is named
"exiv2". On case-sensitive systems (all the Unix systems by default)
trying to look for it as "Exiv2" does not work.

Hence properly look for "exiv2", and adapt the associated CMake
variables accoding to that.

CMakeLists.txt

@@ -40,8 +40,8 @@ if (${QT_VERSION_MAJOR} EQUAL "6")
 endif ()
 
 if (EXIV2_METADATA_SUPPORT)
-    find_package(Exiv2)
-    set_package_properties(Exiv2 PROPERTIES
+    find_package(exiv2)
+    set_package_properties(exiv2 PROPERTIES
         URL "https://www.exiv2.org"
         DESCRIPTION "image metadata support"
         TYPE OPTIONAL
@@ -134,7 +134,7 @@ if (${QT_VERSION_MAJOR} EQUAL "6")
     target_link_libraries (${EXE_NAME} Qt::SvgWidgets)
 endif ()
 
-if (Exiv2_FOUND)
+if (exiv2_FOUND)
     if(NOT TARGET Exiv2::exiv2lib AND TARGET exiv2lib)
         # for exiv2 0.27.x
         add_library(Exiv2::exiv2lib ALIAS exiv2lib)
@@ -143,7 +143,7 @@ if (Exiv2_FOUND)
         Exiv2::exiv2lib
     )
     target_compile_definitions(${EXE_NAME} PRIVATE
-        HAVE_EXIV2_VERSION="${Exiv2_VERSION}"
+        HAVE_EXIV2_VERSION="${exiv2_VERSION}"
     )
 endif ()