Fix AC_PATH_PROG() calls to specify default.
Fix AC_PATH_PROG() calls to specify default.

Fix AC_PATH_PROG() calls to specify 'false' as the default, so that if
the configure script fails to find a program, the false program is
instead called, and an error occurs.  Prior to this fix, if xsltproc
could not be found, make would not report an error due to the leading -o
in the xsltproc invocation.

Reported by David Reiss.

flyfeifan/jemalloc,android-ia/platform_external_jemalloc,geekboxzone/mmallow_external_jemalloc,tempbottle/jemalloc,lilothar/jemalloc,percona/jemalloc-debian,lvfeng1130/jemalloc,geekboxzone/lollipop_external_jemalloc,flyfeifan/jemalloc,yy-nm/jemalloc,AndroidExternal/jemalloc,CyanogenMod/android_external_jemalloc,xin3liang/platform_external_jemalloc,yy-nm/jemalloc,xin3liang/platform_external_jemalloc,glandium/jemalloc,kmiku7/jemalloc,wqfish/jemalloc,jemalloc/jemalloc-cmake,android-ia/platform_external_jemalloc,CyanogenMod/android_external_jemalloc,wfxiang08/jemalloc,TeamExodus/external_jemalloc,CMRemix/android_external_jemalloc,sudosurootdev/external_jemalloc,cpeterso/jemalloc,geekboxzone/lollipop_external_jemalloc,lilothar/jemalloc,wespelee/jemalloc,shekkbuilder/jemalloc,garoose/jemalloc,TeamExodus/external_jemalloc,kmiku7/jemalloc,garoose/jemalloc,Snaipe/jemalloc,ahmetmircik/jemalloc,awreece/jemalloc,wqfish/jemalloc,wespelee/jemalloc,garoose/jemalloc,Snaipe/jemalloc,android-ia/platform_external_jemalloc,leebaok/jemalloc-4.2.1-readcode,jemalloc/jemalloc-cmake,shekkbuilder/jemalloc,yy-nm/jemalloc,percona/jemalloc-debian,8l/jemalloc,geekboxzone/mmallow_external_jemalloc,tempbottle/jemalloc,Snaipe/jemalloc,cpeterso/jemalloc,dxq-git/jemalloc,geekboxzone/mmallow_external_jemalloc,yy-nm/jemalloc,VRToxin/android_external_jemalloc,leebaok/jemalloc-4.2.1-readcode,flyfeifan/jemalloc,CyanogenMod/android_external_jemalloc,xin3liang/platform_external_jemalloc,dxq-git/jemalloc,VRToxin/android_external_jemalloc,shekkbuilder/jemalloc,wqfish/jemalloc,sudosurootdev/external_jemalloc,Dmitry-Me/jemalloc,CMRemix/android_external_jemalloc,VRToxin/android_external_jemalloc,percona/jemalloc-debian,android-ia/platform_external_jemalloc,xin3liang/platform_external_jemalloc,8l/jemalloc,AndroidExternal/jemalloc,glandium/jemalloc,Snaipe/jemalloc,tempbottle/jemalloc,vashstorm/jemalloc,leebaok/jemalloc-4.2.1-readcode,ahmetmircik/jemalloc,cpeterso/jemalloc,sudosurootdev/external_jemalloc,8l/jemalloc,dxq-git/jemalloc,wfxiang08/jemalloc,CMRemix/android_external_jemalloc,jemalloc/jemalloc-cmake,wfxiang08/jemalloc,percona/jemalloc-debian,tempbottle/jemalloc,wqfish/jemalloc,vashstorm/jemalloc,AndroidExternal/jemalloc,wfxiang08/jemalloc,kmiku7/jemalloc,AndroidExternal/jemalloc,CMRemix/android_external_jemalloc,flyfeifan/jemalloc,jemalloc/jemalloc-cmake,Dmitry-Me/jemalloc,shekkbuilder/jemalloc,Dmitry-Me/jemalloc,dxq-git/jemalloc,wespelee/jemalloc,geekboxzone/lollipop_external_jemalloc,vashstorm/jemalloc,lilothar/jemalloc,ahmetmircik/jemalloc,geekboxzone/lollipop_external_jemalloc,vashstorm/jemalloc,awreece/jemalloc,wqfish/jemalloc,lvfeng1130/jemalloc,geekboxzone/mmallow_external_jemalloc,Dmitry-Me/jemalloc,TeamExodus/external_jemalloc,lvfeng1130/jemalloc,awreece/jemalloc,TeamExodus/external_jemalloc,glandium/jemalloc,lilothar/jemalloc,awreece/jemalloc,sudosurootdev/external_jemalloc,ahmetmircik/jemalloc,kmiku7/jemalloc,VRToxin/android_external_jemalloc,CyanogenMod/android_external_jemalloc,wespelee/jemalloc,cpeterso/jemalloc,8l/jemalloc,garoose/jemalloc,glandium/jemalloc,leebaok/jemalloc-4.2.1-readcode,lvfeng1130/jemalloc