Merge bitcoin-core/secp256k1#1090: configure: Remove redundant pkg-config code

21b2ebaf74 configure: Remove redundant pkg-config code (Tim Ruffing)

Pull request description:

  This removes code that detects the pkg-config tool. We used this
  back in the days when we had dependencies. ;) It can always be brought
  back if we'll need it in the future.

  Note that we still deliver a .pc file for this library, and there is
  code in Makefile.am to install it. But this does not require the
  pkg-config tool; only consumers of the .pc file will need it. This can
  be verified by running `make install` (maybe after `mkdir /tmp/pre` and
  `./configure --prefix=/tmp/pre` and checking that the .pc file is
  installed correctly.

ACKs for top commit:
  theuni:
    ACK 21b2ebaf74.
  fanquake:
    ACK 21b2ebaf74

Tree-SHA512: 07affcd0e85f59d10479f279c832b1384208bead2fd152e0d1e3d99167dba4e14dbe87b0bc9c367f0f18da3d37f1d51de064689bff329ee5b01cacfe54e5ede7

configure.ac

@@ -32,8 +32,6 @@ AM_INIT_AUTOMAKE([1.11.2 foreign subdir-objects])
 # Make the compilation flags quiet unless V=1 is used.
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
-PKG_PROG_PKG_CONFIG
-
 AC_PROG_CC
 if test x"$ac_cv_prog_cc_c89" = x"no"; then
   AC_MSG_ERROR([c89 compiler support required])
@@ -408,11 +406,6 @@ AC_SUBST(LIB_VERSION_CURRENT, _LIB_VERSION_CURRENT)
 AC_SUBST(LIB_VERSION_REVISION, _LIB_VERSION_REVISION)
 AC_SUBST(LIB_VERSION_AGE, _LIB_VERSION_AGE)
 
-# Make sure nothing new is exported so that we don't break the cache.
-PKGCONFIG_PATH_TEMP="$PKG_CONFIG_PATH"
-unset PKG_CONFIG_PATH
-PKG_CONFIG_PATH="$PKGCONFIG_PATH_TEMP"
-
 AC_OUTPUT
 
 echo