fix: Use PkgConfig for WebP and WebM (#1400)

CMakeLists.txt

@@ -106,7 +106,8 @@ if(SD_WEBP)
           "Or link against system library:\n  cmake (...) -DSD_USE_SYSTEM_WEBP=ON")
     endif()
     if(SD_USE_SYSTEM_WEBP)
-        find_package(WebP REQUIRED)
+        find_package(WebP)
+        if(WebP_FOUND)
             add_library(webp ALIAS WebP::webp)
             # libwebp CMake target naming is not consistent across versions/distros.
             # Some export WebP::libwebpmux, others export WebP::webpmux.
@@ -120,6 +121,14 @@ if(SD_WEBP)
                     "Expected WebP::libwebpmux or WebP::webpmux."
                 )
             endif()
+        else()
+            find_package(PkgConfig REQUIRED)
+            pkg_check_modules(WebP REQUIRED IMPORTED_TARGET GLOBAL libwebp)
+            pkg_check_modules(WebPMux REQUIRED IMPORTED_TARGET GLOBAL libwebpmux)
+            link_libraries(PkgConfig::WebP)
+            link_libraries(PkgConfig::WebPMux)
+            add_library(libwebpmux ALIAS PkgConfig::WebPMux)
+        endif()
     endif()
 endif()
 
@@ -133,6 +142,13 @@ if(SD_WEBM)
           "Or link against system library:\n  cmake (...) -DSD_USE_SYSTEM_WEBM=ON")
     endif()
     if(SD_USE_SYSTEM_WEBM)
+        find_package(PkgConfig)
+        if(PkgConfig_FOUND)
+            pkg_check_modules(WebM REQUIRED IMPORTED_TARGET GLOBAL libwebm)
+        endif()
+        if(PkgConfig_FOUND AND WebM_FOUND)
+            link_libraries(PkgConfig::WebM)
+        else()
             find_path(WEBM_INCLUDE_DIR
                 NAMES mkvmuxer/mkvmuxer.h mkvparser/mkvparser.h common/webmids.h
                 PATH_SUFFIXES webm
@@ -147,6 +163,7 @@ if(SD_WEBM)
                 INTERFACE_INCLUDE_DIRECTORIES "${WEBM_INCLUDE_DIR}")
         endif()
     endif()
+endif()
 
 set(SD_LIB stable-diffusion)