Add new module to check if a library is installed.

This is very suboptimal compared to using pkg-config, but there are
still some libraries which don't install pkg-config files.

chocpkg/modules.sh

@@ -21,6 +21,10 @@ check_pkgconfig() {
     install_module check_pkgconfig "$@"
 }
 
+check_library() {
+    install_module check_library "$@"
+}
+
 check_tool() {
     install_module check_tool "$@"
 }

chocpkg/modules/check_library.sh

@@ -0,0 +1,14 @@
+
+check_library::init() {
+    PACKAGE_INSTALLED_LIB=$1
+}
+
+# Function that returns true if a specified C library is installed.
+do_check() {
+  # TODO: A better way to choose compiler command?
+  ${CC:-gcc} \
+      -l$PACKAGE_INSTALLED_LIB $LDFLAGS \
+      -o/dev/null \
+      -x c <( echo "int main() {}")
+}
+