Switch back to the default pkg-config error text

The original issue (#183) appears to be some sort of syntax error,
likely from not having pkg-config installed at the time of generating
the configure script.  Specifying our own error text won't improve that
problem.

This reverts commit ad9f74b962.

configure.ac

@@ -86,7 +86,7 @@ AC_ARG_ENABLE([icb-exmm],
               [AS_VAR_SET([enable_icb_exmm], [no])])
 
 if test x"$enable_icb_exmm" != x"no"; then
-	PKG_CHECK_MODULES([EIGEN3], [eigen3 >= 3.3], [], [AC_MSG_ERROR([Eigen not found. Give hints with PKG_CONFIG_PATH. Disable the need of this dependency using --disable-icb-exmm])])
+	PKG_CHECK_MODULES([EIGEN3], [eigen3 >= 3.3])
 	AC_LANG_PUSH([C++])
 	CPPFLAGS_SAVE=$CPPFLAGS
 	CPPFLAGS=$EIGEN3_CFLAGS