build: fix whitespace in pkg-config variable

Useful for PKG_CONFIG="pkg-config --static"

configure.ac

@@ -526,7 +526,7 @@ BITCOIN_QT_INIT
 
 if test x$use_pkgconfig = xyes; then
 
-  if test x$PKG_CONFIG == x; then
+  if test x"$PKG_CONFIG" == "x"; then
     AC_MSG_ERROR(pkg-config not found.)
   fi
 

src/m4/bitcoin_qt.m4

@@ -86,9 +86,6 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
   fi
 
   if test x$use_pkgconfig = xyes; then
-    if test x$PKG_CONFIG == x; then
-      AC_MSG_ERROR(pkg-config not found.)
-    fi
     BITCOIN_QT_CHECK([_BITCOIN_QT_FIND_LIBS_WITH_PKGCONFIG([$2])])
   else
     BITCOIN_QT_CHECK([_BITCOIN_QT_FIND_LIBS_WITHOUT_PKGCONFIG])