build: Add friendly warning to advise user to install pkg-config

If we can detect the presence of '*curses*.pc' files in some
pkg-config default search directories, print a warning message.

Signed-off-by: Kang-Che Sung <[email protected]>

configure.ac

@@ -565,6 +565,36 @@ esac
 
 case ${htop_curses_capability}-$enable_unicode in
 none-*|nonwide-yes)
+   if test "x$PKG_CONFIG" = x; then (
+      # Friendly warning to advise user to install pkg-config.
+      # The local variables get discarded when done.
+      list=/usr/lib/pkgconfig
+      if test "$cross_compiling" != yes; then
+         case $host_os in
+         darwin*)
+            list="/opt/homebrew/Library/Homebrew/os/mac/pkgconfig $list"
+            ;;
+         freebsd*)
+            list="/usr/libdata/pkgconfig $list"
+            ;;
+         netbsd*)
+            list="/usr/pkg/lib/pkgconfig $list"
+            ;;
+         esac
+      fi
+      if test "x$host_alias" != x; then
+         list="/usr/lib/${host_alias}/pkgconfig $list"
+      fi
+
+      for d in $list; do
+         result=`find "$d" -name '*curses*.pc' 2>/dev/null | sed '1 q'`
+         if test "x$result" != x; then
+            AC_MSG_WARN([your system supports pkg-config; installing pkg-config is recommended before configuring htop])
+            break
+         fi
+      done
+   ) fi
+
    # OpenBSD and Solaris are known to not provide '*curses*.pc' files.
    AC_MSG_RESULT([no curses information found through '*-config' utilities; will guess the linker flags])
    for curses_name in $curses_pkg_names; do