cmake: Add modules for curl and alsa

Older versions of cmake (e.g. in Ubuntu 18.04) did not provide cmake
modules for these libraries that also created a target.

Fixes #2393

cmake/libpoly.cmake

@@ -50,12 +50,10 @@ find_package(CairoFC REQUIRED)
 
 if (ENABLE_ALSA)
   find_package(ALSA REQUIRED)
-  set(ALSA_VERSION ${ALSA_VERSION_STRING})
 endif()
 
 if (ENABLE_CURL)
   find_package(CURL REQUIRED)
-  set(CURL_VERSION ${CURL_VERSION_STRING})
 endif()
 
 if (ENABLE_MPD)

cmake/modules/FindALSA.cmake

@@ -0,0 +1,14 @@
+# This module defines an imported target `ALSA::ALSA` if alsa is found
+#
+# Defines the following Variables (see find_package_impl for more info):
+# ALSA_FOUND
+# ALSA_INCLUDE_DIR
+# ALSA_INCLUDE_DIRS
+# ALSA_LIBRARY
+# ALSA_LIBRARIES
+# ALSA_VERSION
+find_package_impl("alsa" "ALSA" "alsa/asoundlib.h")
+
+if(ALSA_FOUND AND NOT TARGET ALSA::ALSA)
+  create_imported_target("ALSA::ALSA" "${ALSA_INCLUDE_DIR}" "${ALSA_LIBRARY}")
+endif()

cmake/modules/FindCURL.cmake

@@ -0,0 +1,14 @@
+# This module defines an imported target `CURL::libcurl` if libcurl is found
+#
+# Defines the following Variables (see find_package_impl for more info):
+# CURL_FOUND
+# CURL_INCLUDE_DIR
+# CURL_INCLUDE_DIRS
+# CURL_LIBRARY
+# CURL_LIBRARIES
+# CURL_VERSION
+find_package_impl("libcurl" "CURL" "curl/curl.h")
+
+if(CURL_FOUND AND NOT TARGET CURL::libcurl)
+  create_imported_target("CURL::libcurl" "${CURL_INCLUDE_DIR}" "${CURL_LIBRARY}")
+endif()